THE  EVOLUTION 
OF  WORLDS 


JL  DWELL 


THE    EVOLUTION    OF   WORLDS 


THE  MACMILLAN  COMPANY 

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SATURN  —  PHOTOGRAPHED  AT  THE  LOWELL  OBSERVATORY 
BY  MR.  E.  C.  SLIPHER.     SEPTEMBER,  1909. 


THE  EVOLUTION   OF 
WORLDS 


BY 


PERGIVAL   LOWELL,  A.B.,  LL.D. 

AUTHOR  OP  "MARS  AND  ITS  CANALS,"  "MARS  AS  THE 

ABODE  OF  LIFE,"  ETC. 
DIRECTOR  OF  THE  OBSERVATORY  AT  FLAGSTAFF,  ARIZONA;  NON-RESIDENT  PROFESSOR 

OF   ASTRONOMY  AT  THE   MASSACHUSETTS   INSTITUTE  OF  TECHNOLOGY;   FELLOW  OF 
THE   AMERICAN   ACADEMY    OF   ARTS    AND    SCIENCES?    MEMBRE    DE    LA    SOCIKTE 
ASTRONOMIQUE   DE   FRANCE?    MEMBER   OF  THE   ASTRONOMICAL  AND  ASTRO- 
PHYSICAL  SOCIETY  OF  AMERICA;  MITGLIED  DER  ASTRONOMISCHE  GE- 
SELLSCHAFT;    MEMBRE   DE    LA    SOCIETE    BELGE    D'ASTRONOMIE; 
HONORARY  MEMBER  OF  THE  SOCIEDAD  ASTRONOMICA  DE  MEX- 
ICO; JANSSEN  MEDALLIST  OF  THE  SOCIETE  ASTRONOMIQUE  DE 
FRANCE,  1904,  FOR  RESEARCHES  ON  MARS?  MEDALLIST 

OF   THE   SOCIEDAD   ASTRONOMICA    DE   MEXICO 
FOR  STUDIES   ON  MARS,  1908 


ILLUSTRATED 


Nefo  gorfc 
THE    MACMILLAN    COMPANY 

1910 

All  rights  reserved 


COPYRIGHT,   1909, 
BY   THE    MACMILLAN    COMPANY. 


Set  up  and  electrotyped.    Published  December,  1909.     Reprinted 
May,  igia 


J.  S.  Cushing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


CO 

THE    PRESIDENT    OF    THE    MASSACHUSETTS    INSTITUTE 

OF    TECHNOLOGY 

TO    MY    COLLEAGUES    THERE 

AND     TO     ITS     STUDENT     BODY 

TO    WHOSE    INTEREST    AND    ATTENTION    THESE 

LECTURES    ARE    INDEBTED 
THEY    ARE    APPRECIATIVELY    INSCRIBED 


"  Si  je  n'etais  pas  devenu  general  en  chef  et  1'instrument  du  sort  d'un 
grand  people,  j'aurais  couru  les  bureaux  et  les  salons  pour  me  mettre 
dans  la  dependance  de  qui  que  ce  fut,  en  qualite  de  ministre  ou  d'am- 
bassadeur?  Non,  non!  je  me  serais  jete  dans  1' etude  des  sciences 
exactes.  J'aurais  fait  mon  chemin  dans  la  route  des  Galilee,  des 
Newton.  Et  puisque  j'ai  reussi  constamment  dans  mes  grandes  entre- 
prises,  eh  bien,  je  me  serais  hautement  distingue  aussi  par  des  travaux 
scientifiques.  J'aurais  laisse  le  souvenir  de  belles  decouvertes.  Aucune 
autre  gloire  n'aurait  pu  tenter  mon  ambition." 

—  NAPOLEON   IHR,  QUOTED  BY  ARAGO. 


THE  substance  of  the  following  pages  was  written 
and  presented  in  a  university  course  of  lectures  before 
the  Massachusetts  Institute  of  Technology  —  in  Feb- 
ruary and  March  of  this  year.  The  kind  interest 
with  which  the  lectures  were  received,  not  only  by  the 
students  and  professional  bodies,  but  by  the  public, 
was  followed  by  an  immediate  request  from  The  Mac- 
millan  Company  to  issue  them  in  book  form,  and  as 

such  they  now  appear. 

PERCIVAL   LOWELL. 

BOSTON,   MASS.,  May  29,  1909. 


vH 


CONTENTS 

CHAPTER  PAGE 

I.      BIRTH  OF  A  SOLAR  SYSTEM    .....  i 

II.      EVIDENCE  OF  THE  INITIAL  CATASTROPHE  IN  OUR  OWN 

CASE      .          .          .  .  „  .  .          .31 

III.  THE  INNER   PLANETS      .  .  .          .          .          .58 

IV.  THE   OUTER  PLANETS    .  .  .          .          .94 
V.      FORMATION  OF  PLANETS           .          .          .          .  .127 

VI.      A   PLANET'S  HISTORY — SELF-SUSTAINED  STAGE  .      155 

VII.      A   PLANET'S  HISTORY  —  SUN-SUSTAINED  STAGE  .          .      182 

VIII.      DEATH  OF  A  WORLD     .          .          .  .          .213 

NOTES 

1.  METEOR  ORBITS          .......      241 

2.  DENSITIES  OF  THE  PLANETS  ......      243 

3.  VARIATION  IN  SPECTROSCOPIC  SHIFT        ....      243 

4.  ON  THE  PLANETS'    ORBITAL  TILTS          ....      244 

5.  PLANETS  AND  THEIR  SATELLITE  SYSTEMS  .          .          .245 

6.  ON     THE    INDUCED     CIRCULARITY    OF     ORBITS    THROUGH 

COLLISION    .          .          .          .          .          .          .          .250 

7.  CAPTURE  OF  SATELLITES       .          .          .          .          .          .251 

INDEX/- 253 


ix 


LIST   OF    ILLUSTRATIONS 


PLATES 
I.      Saturn .    Frontispiece 

OPPOSITE   PAGE 

II.      The  Moving  Nebula  surrounding  Nova  Persei,  1901  — 

1902     .          .  .          .          .          .          .          .14 

III.  Representative  Stellar  Spectra .  .  .          .          .24 

IV.  Spectra  of  the  Major  Planets    .           .           .          .          .52 
V.      Venus,  1896-1897 82 

VI.  Asteroids :   Major  Axes  of  Orbits      ....        98 

VII.  Saturn  —  A  Drawing  showing  Agglomerations     .  .108 

VIII.  Spectrogram  of  Jupiter,  Moon  Comparison  .          .      152 

IX.  Spectrogram  showing  Water- vapor  in  Atmosphere  of  Mars     1 60 

X.  Tree  Fern  .          .          .          .          .          .          .          .176 

XI.  Ten  Views  of  Mercury,  showing  Effect  of  Libration     .      222 

XII.  Spectrogram  of  Saturn    .          .          .          .          .          .232 

CUTS    APPEARING   IN   TEXT 

PAGE 

Algol  and  its  Dark  Companion   ......          4 

Nova  Persei    .          .          .          .          .          .          .          .          .11 

Spectrum  of  Nova  Persei  .          .          .          .          .          .          .12 

The  Moving  Nebula  surrounding  Nova  Persei,  1 90 1        .          .        13 
Great'Nebula  in  Orion      .          .          .          .          .          .          .17 

Great  Nebula  in  Andromeda       .          .          .          .          .          .18 

Nebula  M.   100  Comae     .  .          .          .          .          .        19 

Nebula  ijl  I.  226  Ursae  Majoris  .....        20 

Nebula  i$  V.  24  Comae.      Showing  Globular  Structure    .  .        21 

xi 


xii  LIST   OF   ILLUSTRATIONS 

PAGE 

Nebula  M.  101  Ursae  Majoris    .          .          .          .          „          .23 

The  Radiant  of  a  Meteoric  Shower      •          .          .          .          .        37 

Diagram  explaining  Proportionate  Visibility  of  Meteors     .          .        38 
The  Mart  Iron         .          .  _'•_       ,          .          .          .  .          .        41 

Section  of  Meteorite  showing  Widmannstattian  Lines        .          .        42 
Meteorite,  Toluca   .          ...          .          .          .          .        43 

Nebula  t$  V.  14  Cygni    %         •          •          ....        45 

Nebula  N.G.C.  1499  Persei     .  .          .          .46 

Nebula  N.G.C.  6960  in  Cygnus        .....        47 

Nebula  M.  51  Canum  Venaticorum    .....        48 

Orbits  of  the  Inner  Planets          .          .          .          .          .          -59 

Sulla  Rotazione  di  Mercurio.  —  Di  G.  V.  Schiaparelli     .  .        64 

Map  of  Mercury.      Lowell         ......        69 

Venus.      October,  i896-March,  1897         ....        78 

Venus.      April  12,  1909.          ......        79 

Diagram :   Convection  Currents  in  Atmosphere  of  Venus  .        8 1 

Diagram  :   Shift  in  Central  Barometric  Depression  .          .          .81 
Spectrogram  of  Venus,  showing  its  Long  Day         .          .          .87 
Spectrogram  of  Jupiter,  giving  the  Length  of  its  Day  by  the  Tilt 

of  its  Spectral  Lines  .  .....        89 

Orbits  of  the  Outer  Planets         .          .          .          .          .          •        95 

Drawing  of  Jupiter  showing  its  Ellipticity      .          .          .  .103 

Two  Drawings  of  Jupiter  and  its  Wisps         .          .          .          .105 

Photograph  of  Jupiter,  1909       .          .          .          .          .          .107 

Diagram  of  Saturn's  Rings  .          .          .          .          .          .113 

The  Tores  of  Saturn          .  .          .          .          .          .          .114 

Chart  showing  increasing  Tilts  of  the  Major  Planets  .  .  131 
Orbital  Tilts  and  Eccentricities  of  Satellites  .  .  .  *  !33 
Masses  of  Planets  and  Satellites  .  .  .  ,  ».  .136 
Two  Drawings  of  Jupiter  and  its  "Great  Red  Spot"  .  •  164 
Sun  Spots  .  .  .  "•„•".'.••'•  .  .165 
Photograph  of  a  Sun  Spot  .  .  •  ,  .  .  166 


LIST   OF   ILLUSTRATIONS  xiii 


PAGE 


The  Volcano  Colima,  Mexico,  March  24,  1903   .          .          .169 
Jukes  Butte,  a  Denuded  Laccolith,  as  seen  from  the  Northwest.      170 
Ideal  Section  of  a  Laccolith          .          .  .          .          .          .170 

Earth  as  seen  from   above.  —  Photographed  at   an  Altitude  of 

5500  Feet 183 

Tracks  of  Sauropus  Primaevus      .          .          .          .          .          .188 

Adventures  of  a  Heat  Ray  .          .          .          .          .          .193 

Polar  Caps  of  Mars  at  their  Maxima  and  Minima  .          .          .198 
Glacial  Map  of  Eurasia      .          .          .          .          .          .          .200 

Map  showing  the  Glaciated  Area  of  North  America          .          .201 
Photograph  of  the  Moon   .          .          .          .          .          .          .205 

Petrified  Bridge,  Third  Petrified  Forest,  near  Adamana,  Arizona  210 
Three  Views  of  Venus,  showing  Agreement  at  Different  Distances  220 
Diagram  of  Libration  in  Longitude  due  to  Rotation  .  .  222 

Moon,  —  Full  and  Half 225 

Diagram  illustrating  Molecular  Motion  in  a  Gas  .  .  .227 
Distribution  of  Molecular  Velocities  in  a  Gas  .  .  .229 


THE  EVOLUTION  OF  WORLDS 

CHAPTER   I 

BIRTH    OF    A    SOLAR    SYSTEM 

ASTRONOMY  is  usually  thought  of  as  the  study 
of  the  bodies  visible  in  the  sky.  And  such  it 
largely  is  when  the  present  state  of  the  universe  alone 
is  considered.  But  when  we  attempt  to  peer  into  its 
past  and  to  foresee  its  future,  we  find  ourselves  facing  a 
new  side  of  the  heavens  —  the  contemplation  of  the 
invisible  there.  For  in  the  evolution  of  worlds  not 
simply  must  the  processes  be  followed  by  the  mind's 
eye,  so  short  the  span  of  human  life,  but  they  begin 
and  end  in  what  we  cannot  see.  What  the  solar 
system  sprang  from,  and  what  it  will  eventually  become, 
is  alike  matter  devoid  of  light.  Out  of  darkness  into 
darkness  again :  such  are  the  bourns  of  cosmic  action. 
The  stars  are  suns;  past,  present,  or  potential.  Each 
of  those  diamond  points  we  mark  studding  the  heavens 
on  a  winter's  night  are  globes  comparable  with,  and 
in  many  cases  greatly  excelling,  our  own  ruler  of  the 
day.  The  telescope  discloses  myriads  more.  Yet 


VOLUTION  OF  WORLDS 

these  self-confessed  denizens  of  space  form  but  a 
fraction  of  its  occupants.  Quite  as  near,  and  perhaps 
much  nearer,  are  orbs  of  which  most  of  us  have  no 
suspicion.  Unimpressing  our  senses  and  therefore 
ignored  by  our  minds,  bodies  people  it  which,  except 
for  rare  occurrences,  remain  forever  invisible.  For 
dark  stars  in  countless  numbers  course  hither  and 
thither  throughout  the  universe  at  speeds  as  stupendous 
as  the  lucent  ones  themselves. 

Had  we  no  other  knowledge  of  them,  reasoning 
would  suffice  to  demonstrate  their  existence.  It  is 
the  logic  of  unlimited  subtraction.  Every  self-shining 
star  is  continually  giving  out  light  and  heat.  Now 
such  an  expenditure  cannot  go  on  forever,  as  the  source 
of  its  replenishing  by  contraction,  accretion,  or  dis- 
integration is  finite.  Long  to  our  measures  of  time  as 
the  process  may  last,  it  must  eventually  have  an  end 
and  the  star  finally  become  a  cold  dark  body,  pursuing 
as  before  its  course,  but  in  itself  inert  and  dead ;  an 
orb  grown  orbed,  in  the  old  French  sense.  So  it  must 
remain  unless  some  cosmic  catastrophe  rekindle  it  to 
life.  The  chance  of  such  occurrence  in  a  given  time 
compared  with  the  duration  of  the  star's  light-emitting 
career  will  determine  the  number  of  dark  stars  relative 
to  the  lucent  ones.  The  chance  is  undoubtedly  small, 
and  the  number  of  dark  bodies  in  space  proportionally 
large.  Reasoning,  then,  informs  us  first  that  such 


BIRTH   OF   A   SOLAR    SYSTEM  3 

bodies  must  exist  all  about  us,  and  second  that  their 
multitude  must  be  great. 

Valid  as  this  reasoning  is,  however,  we  are  not  left 
to  inference  for  our  knowledge  of  them.  There  is 
a  certain  star  amid  the  polar  constellations  known 
as  Algol,  —  el  Ghoul,  the  Arabs  called  it,  or  The 
Daemon.  The  name  shows  they  noticed  how  it  winked 
its  eye  and  recognized  something  sarcastically  sinister 
in  its  intent.  For  once  in  two  days  and  twenty  hours 
its  light  fades  to  one-third  of  its  usual  amount,  remains 
thus  for  about  twenty  minutes,  and  then  slowly  regains 
its  brightness.  Seemingly  unmoved  itself,  its  steady 
blinking  from  the  time  man  first  observed  it  took  on 
an  uncanniness  he  felt.  To  untelescoped  man  it 
certainly  seemed  demoniacal,  this  punctual  recurrent 
wink.  Spectroscoped  man  has  learnt  its  cause. 

Goodricke  in  1795  divined  it,  and  research  since  has 
confirmed  his  keen  intuition.  Its  loss  of  light  is  oc- 
casioned by  the  passing  in  front  of  it  of  a  dark  com- 
panion almost  of  its  own  size  revolving  about  it  in  a 
close  elliptic  orbit.  That  this  is  the  explanation  of 
its  strange  behavior,  the  shift  of  its  spectral  lines  makes 
certain,  by  showing  that  the  bright  star  is  receding 
from  us  at  twenty-seven  miles  a  second  seventeen  hours 
before  the  eclipse  and  coming  towards  us  at  about  the 
same  rate  seventeen  hours  after  it;  its  dark  companion, 
therefore,  doing  the  reverse. 


4  THE  EVOLUTION  OF  WORLDS 

Algol  is  no  solitary  specimen  of  a  mind-seen  invisible 
star.  Many  eclipsing  binaries  of  the  same  class  are 
now  known;  and  considering  that  the  phenomenon 
could  not  be  disclosed  unless  the  orbital  plane  of  the 

,  ALGOL  AND  ITS  DARK  COMPANION, 


P.  L. 


AS  SEEN  FROM  ABOVE  ORBIT. 


pair  traversed  the  observer's  eye,  an  unlikely  chance 
in  a  fortuitous  distribution,  we  perceive  how  many  such 
in  truth  there  must  be  which  escape  recognition  for 
their  tilt. 

But  if  dark  stars  exist  in  connection^ with  lucent  ones, 


BIRTH  OF  A  SOLAR  SYSTEM  5 

there  must  be  many  more  that  travel  alone.  Our  own 
Sun  is  an  instance  in  embryo.  If  he  live  long  enough, 
he  will  become  such  a  solitary  shrouded  tramp  in  his 
old  age.  For  he  has  no  companion  to  betray  him. 
The  only  way  in  which  we  could  become  cognizant  of 
these  wanderers  would  be  by  their  chance  collision 
with  some  other  star,  dark  or  lucent  as  the  case  might 
be.  The  impact  of  the  catastrophe  would  generate 
so  much  light  and  heat  that  the  previously  dark  body 
would  be  converted  into  a  blazing  sun  and  a  new  star 
make  its  advent  in  the  sky. 

Star  births  of  the  sort  have  actually  been  noted. 
Every  now  and  then  a  new  star  suddenly  appears  in 
the  firmament  —  a  nova  as  it  is  technically  called. 
These  apparitions  date  from  the  dawn  of  astronomic 
history.  The  earliest  chronicled  is  found  in  the  Chinese 
Annals  of  134  B.C.  It  shone  out  in  Scorpio  and  was 
probably  the  new  star  which  Pliny  tells  us  incited 
Hipparchus,  "The  Father  of  Astronomy,"  to  make  his 
celebrated  catalogue  of  stars.  From  this  time  down 
we  have  recorded  instances  of  like  character. 

One  of  the  most  famous  was  the  "Pilgrim  Star"  of 
Tycho  Brahe.  That  astronomer  has  left  us  a  full  ac- 
count of  it.  "While  I  was  living,"  he  tells  us,  "with  my 
uncle  in  the  monastery  of  Hearitzwadt,  on  quitting  my 
chemical  laboratory  one  evening,  I  raised  my  eyes  to 
the  well-known  vault  of  heaven  and  observed,  with  in- 


6  THE  EVOLUTION  OF  WORLDS 

describable  astonishment,  near  the  zenith,  in  Cassiopeia, 
a  radiant  fixed  star  of  a  magnitude  never  before  seen. 
In  my  amazement  I  doubted  the  evidence  of  my  senses. 
However,  to  convince  myself  that  it  was  no  illusion,  and 
to  have  the  testimony  of  others,  I  summoned  my  assist- 
ants from  the  laboratory  and  inquired  of  them,  and  of 
all  the  country  people  that  passed  by,  if  they  also  ob- 
served the  star  that  had  thus  suddenly  burst  forth.  I 
subsequently  heard  that  in  Germany  wagoners  and 
other  common  people  first  called  the  attention  of  as- 
tronomers to  this  great  phenomenon  in  the  heavens,  - 
a  circumstance  which,  as  in  the  case  of  non-predicted 
comets,  furnished  fresh  occasion  for  the  usual  raillery 
at  the  expense  of  the  learned." 

The  new  star,  he  informs  us,  was  just  like  all  other 
fixed  stars,  but  as  bright  as  Venus  at  her  brightest. 
Those  gifted  with  keen  sight  could  discern  it  in  the  day- 
time and  even  at  noon.  It  soon  began  to  wane.  In 
December,  1572,  it  resembled  Jupiter,  and  a  year  and 
three  months  later  had  sunk  beyond  recognition  to  the 
naked  eye.  It  changed  color  as  it  did  so,  passing  from 
white  through  yellow  to  red.  In  May,  1573,  it  returned 
to  yellow  ("the  hue  of  Saturn,"  he  expressly  states), 
and  so  remained  till  it  disappeared  from  sight,  scintillat- 
ing strongly  in  proportion  to  its  faintness. 

Thirty-two  years  later  another  stranger  appeared  and 
was  seen  by  Kepler,  who  wrote  a  paper  about  it  en- 


BIRTH  OF  A  SOLAR  SYSTEM  7 

titled  "The  New  Star  in  the  Foot  of  the  Serpent."  It 
shone  out  in  the  same  sudden  manner  and  faded  in  the 
same  leisurely  way. 

Since  1860  there  have  been  several  such  apparitions, 
and  since  1876  it  has  been  possible  to  study  them  with 
the  spectroscope,  which  has  immensely  increased  our 
knowledge  of  their  constitution.  Indeed,  this  instru- 
ment of  research  has  really  opened  our  eyes  to  what  they 
are.  Nova  Cygni,  in  1876,  Nova  Aurigae,  in  1892,  and 
Nova  Persei,  in  1901,  besides  several  others  found  by 
Mrs.  Fleming  on  the  Arequipa  plates,  were  excellent 
examples,  and  all  agreed  in  their  main  features,  showing 
that  novae  constitute  a  type  of  stars  by  themselves, 
whose  appearing  in  the  first  place  and  whose  behavior 
afterwards  prove  them  to  have  started  from  like  cause 
and  to  have  pursued  parallel  lines  of  development. 

As  a  typical  case  we  may  review  the  history  of  Nova 
Aurigae.  On  February  i,  1892,  an  anonymous  post- 
card was  received  by  Dr.  Copeland  of  the  Royal  Ob- 
servatory, Edinburgh,  that  read  as  follows:  "Nova  in 
Aurigae.  In  Milky  Way,  about  2°  south  of  ^  Aurigae, 
preceding  26  Aurigae.  Fifth  magnitude  slightly  brighter 
than  x-"  The  observatory  staff  at  once  looked  for  the 
nova  and  easily  found  it  with  an  opera-glass.  They 
then  examined  it  through  a  prism  placed  before  their 
24-inch  reflector  and  found  its  spectrum.  It  proved  to 
be  that  of  a  "blaze  star." 


8  THE  EVOLUTION  OF  WORLDS 

Dr.  Thomas  D.  Anderson  turned  out  to  be  the  writer 
of  the  anonymous  post-card  —  his  name  modestly  self- 
obliterated  by  the  nova's  light.  He  had  detected  the 
star  on  January  24,  but  had  only  verified  it  as  a  new  one 
on  the  3 1  st.  Harvard  College  Observatory  then  looked 
up  its  archived  plates.  The  plates  showed  that  it  had 
appeared  sometime  between  December  i  and  10.  Its 
maximum  had  been  attained  on  December  20,  after 
which  it  declined,  to  record  apparently  another  maxi- 
mum on  February  3  of  the  3.5  magnitude.  From  this 
time  its  light  steadily  waned  till  on  April  I  it  was  only 

of  the  1 6th  magnitude  or —  of  what  it  had  been. 

IOOOOO 

In  August  it  brightened  again  and  then  waned  once 
more. 

Meanwhile  its  spectrum  underwent  equally  strange 
fluctuations.  At  first  it  exhibited  the  bright  lines 
characteristic  of  the  flaming  red  solar  prominences, 
the  calcium,  hydrogen,  and  helium  lines  flanked  by 
their  dark  correlatives  upon  a  continuous  background, 
showing  that  both  glowing  and  cooler  gases  were  here 
concerned.  The  sodium  lines,  too,  appeared,  like  those 
that  come  out  in  comets  as  they  approach  the  furnace 
of  the  Sun.  An  outburst  such  as  occurs  in  miniature  in 
the  solar  chromosphere  or  outermost  gaseous  layer  of 
the  Sun  was  here  going  on  upon  a  gigantic  scale.  A 
veritable  spectral  chaos  next  supervened,  staying  until 


BIRTH  OF  A  SOLAR  SYSTEM  9 

the  star  had  practically  faded  away.  Then,  on  its  re- 
appearance, in  August,  Holden,  Schaeberle,  and  Camp- 
bell discovered  to  their  surprise  not  what  had  been  at  all, 
but  something  utterly  new :  the  soberly  bright  lines  only 
of  a  nebula.  Finally,  ten  years  later,  January,  1902, 
Campbell  found  its  spectrum  had  become  continuous, 
the  body  having  reverted  to  the  condition  of  a  star. 

Now  how  are  we  to  interpret  these  grandiose  vicissi- 
tudes, visually  and  spectrally  revealed  ?  That  we  wit- 
nessed some  great  catastrophe  is  clear.  The  sudden 
increase  of  light  of  many  thousand  fold  from  invisibility 
to  prominence  shows  that  a  tremendous  cataclysm  oc- 
curred. The  bright  lines  in  the  spectrum  confirm 
it  and  imply  that  vast  upheavals  like  those  that  shake 
the  Sun  were  there  in  progress,  but  on  so  stupendous  a 
scale  that,  if  for  no  other  reason,  we  must  dismiss  the 
idea  that  explosions  alone  can  possibly  be  concerned. 
The  dark  correlatives  of  the  bright  lines  have  been  in- 
terpreted as  indicating  that  two  bodies  were  concerned, 
each  travelling  at  velocities  of  hundreds  of  miles  a 
second.  But  in  Nova  Aurigae  shiftings  of  the  spectral 
lines  implying  six  bodies  at  least  were  recorded,  if  such 
be  attributed  to  motion  in  the  line  of  sight,  and  Vogel 
was- minded  to  throw  in  a  few  planets  as  well  —  as  Miss 
Clerke  pithily  puts  it.  There  is  not  room  for  so  many 
on  the  stage  of  the  cosmic  drama.  Other  causes,  as  we 
now  know,  may  also  displace  the  spectral  lines.  Great 


io          THE  EVOLUTION  OF  WORLDS 

pressure  has  been  shown  to  do  it,  thanks  to  the  labors 
of  Humphreys  and  Mohler  at  Baltimore.  "Anomalous 
refraction"  may  do  it,  as  Professor  Julius  of  Utrecht  has 
found  out.  Finally,  changes  of  density  may  produce 
it,  as  Michelson  has  discovered.  To  these  causes  we 
may  confidently  ascribe  most  of  the  shiftings  in  the 
stellar  spectrum,  for  just  such  forces  must  be  there  at 
work. 

Mr.  Monck  suggested  the  idea  that  new  stars  are  the 
result  of  old  dark  stars  rushing  through  gaseous  fields 
in  space  and  rendered  luminous  by  the  encounter.  See- 
liger  revived  and  developed  this  idea,  which  in  certain 
cases  is  undoubtedly  the  truth.  Probably  this  occurred 
to  the  new  star  of  1885  which  suddenly  blazed  out 
almost  in  the  centre  of  the  great  nebula  in  Andromeda. 
It  behaved  like  a  typical  nova  and  in  due  course  faded 
to  indistinguishability.  Something  like  it  happened, 
too,  in  the  nova  of  1860,  which  suddenly  flared  up  in 
the  star  cluster  80  Messier,  outdoing  in  lustre  the  cluster 
itself,  and  then,  too,  faded  away. 

But  just  as  psychology  teaches  us  that  not  only  do  we 
cry  because  we  are  sorrowful,  but  that  we  are  sorrowful 
because  we  cry,  so  while  a  nova  may  be  made  by  a 
nebula,  no  less  may  a  nebula  be  made  by  a  star. 

Let  us  see  how  this  might  be  brought  about  and  what 
sign  manuals  it  would  present.  Suppose  that  the  two 
bodies  actually  grazed.  Then  the  disruption  would 


BIRTH   OF   A   SOLAR   SYSTEM  n 

affect  the  star's  cuticle,  first  raising  the  outer  parts,  con- 
sisting rather  of  carbon  than  of  the  metals,  since  that 
substance  is  the  lighter,  to  intense  heat  and  the  gases 
about  it  at  the  same  time.  The  glowing  carbon  would 
be  intensely  bright,  and  at  first  its  light  would  over- 
power that  from  the  gases,  and  not  till  its  great  glow  had 


partially  subsided  would  theirs  be  seen.  Then  the  gases, 
hydrogen,  helium,  and  so  forth,  would  make  them- 
selves evident.  Finally  only  the  most  tenuous  ones, 

those  peculiar  to  a  nebula,  would  remain  visible.     After 

t 

which  the  more  solid  particles  due  to  the  disruption 
would  fall  together  and  light  up  again  by  their  individ- 
ual collisions.  Much  the  same  would  result  if  without 
striking  the  stars  passed  close. 


12 


THE   EVOLUTION   OF   WORLDS 


Now  to  put  this  theory  to  the  proof.  In  the  early 
morning  of  the  22d  of  February,  1901,  Dr.  Anderson, 
the  discoverer  of  Nova  Aurigae,  perceived  that  Algol 
had  a  neighbor,  a  star  as  bright  as  itself,  which  had 


HrjHCKHe 


Feb. 


Feb.  28 


Mar.  6 


Mar.  15 


Mar.  28 


SPECTRUM  OF  NOVA  PERSEI.     (F.  Ellerman,  40  in.  Yerkes.) 

never  been  there  before.  Within  twenty-four  hours 
of  its  detection  the  newcomer  rivalled  Capella,  and 
shortly  after  took  rank  as  the  premier  star  of  the  north- 
ern hemisphere.  Its  spectrum  on  the  22d  was  found 
at  Harvard  College  Observatory  to  be  like  that  of  Rigel, 
a  continuous  one  crossed  by  some  thirty  faint  dark  lines. 


BIRTH   OF   A   SOLAR   SYSTEM 


On  the  24th,  however,  so  soon  as  it  began  to  wane,  the 
bright  lines  of  hydrogen  were  conspicuous  with  their 
dark  correlatives,  just  as  they  had  been  with  Nova 
Aurigae  and  other  novae.  At  the  same  time  each  par- 
ticular spectral  line  proved  a  law  unto  itself,  some 
shifted  more  than  others,  thus  negativing  motion  as 


their  only  cause  and  indicating  change  of  pressure  or 
density  as  concerned  concomitants  of  the  affair.  Blue 
emissions  like  those  of  Wolf-Rayet  stars  next  made  their 
appearance;  then  a  band,  found  by  Wright  at  the  Lick 
to  characterize  nebulae,  shone  out,  and  finally  in  July  the 
change  to  a  nebular  spectrum  stood  complete. 

Then  came  what  is  the  most  suggestive  feature  in 
the  whole  event.      On  August  22  and  23  Dr.  Wolf  at 


i4         THE   EVOLUTION   OF   WORLDS 

Konigstahl  took  with  his  then  new  Bruce  objective 
some  long  exposure  plates  of  the  nova,  and  on  them 
found,  to  his  surprise,  wisps  of  nebulous  matter  to  the 
southeast  of  the  star.  On  September  20  Ritchey, 
with  a  two-foot  mirror  of  his  own  constructing  exposed 
for  four  hours,  brought  the  whole  formation  to  light. 
It  turned  out  to  be  a  spiral  nebula  encircling  and  ap- 
parently emanating  from  the  star.  Its  connection  with 
the  nova  was  patent.  But  there  was  more  to  come. 
Later  plates  taken  at  the  Lick  on  November  7  dis- 
closed the  startling  fact  that  the  nebula  was  visibly 
expanding,  uncoiling  outward  from  the  star.  A  plate 
by  Ritchey  on  November  13  confirmed  this,  and  still 
later  plates  by  him  in  December,  January,  and  Febru- 
ary showed  the  motion  to  be  progressive.  At  the  same 
time  the  star  showed  no  parallax,  and  the  speed  of  the 
motion  seemed  thus  to  be  indicated  as  enormous. 
Kapteyn  suggested  to  account  for  it  that  appearance, 
not  reality,  was  here  concerned;  that  the  nebula  had 
always  existed,  and  was  only  shown  up  by  the  light 
from  the  conflagration  travelling  outward  from  the 
nova  at  the  rate  of  one  hundred  and  eighty-six  thousand 
miles  a  second.  This  would  make  the  catastrophe  to 
have  occurred  as  far  back  as  the  time  of  James  I,  of 
which  the  news  more  truthful  but  less  timely  than  that 
of  the  morning  papers  had  only  just  reached  us. 

But  a  little  of  that  simple  reasoning  by  which  Zadig 


BIRTH   OF   A   SOLAR    SYSTEM  15 

recovered  the  lost  horses  of  the  Sultan,  and  which  from 
its  unaccustomedness  in  the  affairs  of  men  got  him  sus- 
pected of  having  stolen  them  and  very  nearly  caused 
his  death,  will  show  the  untenableness  of  this  idea  and 
help  us  to  a  solution.  In  the  first  place  we  note  that 
the  star  holds  the  very  centre  of  the  nebular  stage,  a 
remarkable  prominence  if  the  star  has  no  creative  right 
to  the  position.  Then  the  same  knots  and  patches  of 
the  nebulous  configuration  are  visible  in  all  the  photo- 
graphs, in  the  same  relative  positions,  turned  through 
corresponding  angles  as  one  will  see  for  himself,  all 
having  moved  symmetrically  from  one  date  to  another. 
At  the  truly  marvellous  mimicry  implied  if  different 
objects  were  concerned  common  sense  instinctively 
shies,  and  very  properly,  as  the  chances  against  it  are 
millions  to  one.  Clearly  it  was  not  a  mere  matter 
of  ethereal  motion,  but  a  very  material  motion  of 
matter,  which  was  here  concerned.  Something  corpus- 
cular emanating  from  the  nova  spread  outward  into 
space. 

Clinching  this  conclusion  is  the  result  of  a  search  by 
Perrine  for  traces  of  the  nebula  on  earlier  plates.  For 
on  one  taken  by  him  on  March  29  (1901)  he  found  the 
process  already  started  in  two  close  coils,  its  conception 
thus  clearly  dating  from  the  time  of  the  star's  outburst. 
In  Nova  Persei,  then,  we  actually  witnessed  a  spiral 
nebula  evolved  from  a  disrupted  star. 


1 6         THE   EVOLUTION   OF   WORLDS 

What  was  this  ejectum  and  what  drove  it  forth  ? 
Professor  Very  regarded  it  as  composed  of  corpuscles 
such  as  give  rise  to  cathode  rays  discharged  from  the 
star  under  the  stress  of  light  pressure  or  electric  repul- 
sion. But  I  think  we  may  see  in  it  something  simpler 
still;  to  wit,  gaseous  molecules  driven  off  by  light 
pressure  alone  —  the  smoke,  as  one  may  say,  of  the 
catastrophe  —  akin  exactly  to  the  constituents  of  com- 
et's tails.  The  mere  light  of  the  conflagration  pushed 
the  hydrogen  molecules  away.  This  would  explain 
their  presence  and  their  exceeding  hurry  at  the  same 
time.  They  were  started  on  their  travels  by  domestic 
jars  and  kept  going  by  the  vivid  after-effects  of  that 
infelicity. 

The  fairly  steady  rate  of  regression  from  the  nova 
observed  may  be  explained  by  the  observed  decrease  in 
the  light  of  the  repellent  source.  Such  combined  with 
the  retarding  effect  of  gravity  might  make  the  regres- 
sion equable.  This  is  the  more  explanatory  as  the 
speed  was  certainly  much  less  than  that  of  light,  though 
greatly  exceeding  any  possible  from  the  direct  disrup- 
tion. At  the  same  time  both  the  bright  and  the  dark 
lines  of  hydrogen  seen  in  the  spectrum  stand  accounted 
for;  the  colliding  molecules,  at  their  starting  on  their 
travels  from  the  star,  shining  through  their  sparser 
fellows  farther  out.  An  interesting  biograph  of  the 
levity  of  light ! 


BIRTH   OF   A   SOLAR   SYSTEM  17 

Nova  Persei  thus  introduces  us  at  its  birth  to  one  of 
a  class  of  most  interesting  objects  comparatively  re- 


GREAT  NEBULA  IN  ORION  —  AFTER  RITCHEY. 

cently  discovered  and  of  most  pregnant  import,  —  the 
spiral  nebulae. 

In  1843  when  Lord  Rosse's  giant  speculum,  six  feet 


1 8         THE   EVOLUTION  OF  WORLDS 

across,  was  turned  upon  the  sky,  a  nebula  was  brought 
to  light  which  was  unlike  any  ever  before  seen.     It  was 


GREAT  NEBULA  IN  ANDROMEDA  —  AFTER  RITCHEY. 

neither  irregular  like  the  great  nebula  in  Orion  nor  round 
like  the  so-called  planetary  nebulae,  —  the  two  great 
classes  at  that  time  known,  —  but  exhibited  a  striking 


BIRTH   OF   A   SOLAR   SYSTEM  19 

spiral  structure.  It  proved  the  forerunner  of  a  remark- 
able revelation.  For  the  specimen  thus  disclosed  has 
turned  out  to  typify  not  only  the  most  interesting  form 
of  those  heavenly  wreaths  of  light,  but  by  far  the  com- 
monest as  well.  As  telescopic  and  especially  photo- 


NEBULA  M.  100  COMIE —  AFTER  ROBERTS. 

graphic  means  improved,  the  number  of  such  objects 
detected  steadily  increased  until  about  thirteen  years 
ago  Keeler  by  his  systematic  discoveries  of  them  came 
to  the  conclusion  that  a  spiral  structure  pervaded  the 
great  majority  of  all  the  nebulae  visible.  Their  relative 
universality  was  outdone  only  by  the  invariability  of 
their  form.  For  they  all  represent  spirals  of  one  type: 


20         THE   EVOLUTION  OF   WORLDS 

two  coiled  arms  radiating  diametrically  from  a  central 
nucleus  and  dilating  outward.  Even  nebulae  not 
originally  supposed  spiral  have  disclosed  on  better  reve- 
lation the  dominant  form.  Thus  the  great  nebula  in 
Andromeda  formerly  thought  lens-shaped  proves  to  be 


NEBULA  $  I.  226  URSJE  MAJORIS  —  AFTER  ROBERTS. 


a  huge  spiral  coiled  in  a  plane  not  many  degrees  in- 
clined to  the  plane  of  sight. 

As  should  happen  if  the  spirals  are  unrelated,  left- 
handed  and  right-handed  ones  are  about  equally  com- 
mon. In  Dr.  Roberts'  great  collection  of  those  in 
which  the  structure  is  distinctly  discernible,  nine  are 
right-handed,  ten  left-handed,  showing  that  they  par- 
take of  the  ambidextrous  impartiality  of  space. 

Lastly   the   spirals    are   evidently   thicker   near   the 


BIRTH   OF   A   SOLAR   SYSTEM          21 

centre,  thinning  out  at  the  edge,  and  when  the  central 
nucleus  is  pronounced,  it  seems  to  have  a  certain  globu- 
larity  not  shared  by  the  arms,  and  more  or  less  de- 
tached from  them.  This  appears  in  those  cases  where 


NEBULA  #  V.  24  COM^E  —  AFTER  ROBERTS. 
Showing  globular  structure. 

they  are  shown  us  edgewise,  and  it  has  been  thought 
perceptible  in  the  great  nebula  of  Andromeda.  The 
difficulty  in  establishing  the  phenomenon  comes  from 
the  impossibility  of  both  features  showing  at  their  best 
together.  For  the  globularity  to  come  out  well,  the 
spiral  must  be  presented  to  us  nearly  in  the  plane  of 
sight;  for  the  spirality,  in  a  plane  at  right  angles  to  it. 


22         THE   EVOLUTION   OF   WORLDS 

Much  may  be  learnt  by  pondering  on  these  peculi- 
arities. The  widespread  character  of  the  phenomenon 
points  to  some  universal  law.  We  are  here  clearly 
confronted  by  the  embodiment  of  a  great  cosmic  princi- 
ple, causing  the  helices  it  is  for  us  to  uncoil.  It  is  a 
problem  in  mechanics. 

In  the  first  place,  a  spiral  structure  denotes  action 
on  the  face  of  it.  It  implies  a  rotation  combined 
with  motion  out  or  in.  We  are  familiar  with  the 
fact  in  the  sparks  of  pin-wheel  pyrotechnics.  Any 
rotating  fluid  urged  by  an  outward  or  an  inward 
impulse  must  take  the  spiral  form.  A  common  ex- 
ample occurs  in  the  water  let  out  of  a  basin  through  a 
hole  in  the  centre  when  we  draw  out  the  plug.  Here 
the  force  is  inward,  and  because  the  bowl  and  orifice 
are  not  perfectly  symmetric,  a  rotation  is  set  up  in  the 
water  trying  to  escape,  and  the  two  combine  to  give  us 
a  beautiful  conchoidal  swirl.  In  this  case  the  particles 
seek  the  centre,  but  the  same  general  shape  is  assumed 
when  they  seek  to  leave  it. 

Another  point  to  be  noticed  is  that  a  spiral  nebula 
could  not  develop  of  itself  and  subsist.  To  continue 
it  must  have  outside  help.  For  if  it  were  due  to  in- 
ternal explosive  action  in  the  pristine  body,  each  ejec- 
tum  must  return  to  the  point  it  started  from,  or  else 
depart  forever  into  space,  for  the  orbit  it  would  de- 
scribe must  either  be  closed  or  unclosed.  If  the  former, 


BIRTH   OF   A   SOLAR   SYSTEM  23 

it  would  revisit  its  starting-point;  if  the  latter,  it  would 
never  return.     Explosion,  therefore,  of  itself  could  not 


NEBULA  M.  101  URS.E  MAJORIS  —  AFTER  RITCHEY. 

have  produced  the  forms  we  see,  unless  they  be 
ephemeral  apparitions,  a  supposition  their  presence 
throughout  the  heavens  seems  effectually  to  exclude. 


24         THE   EVOLUTION   OF   WORLDS 

The  form  of  the  spiral  nebulae  proclaims  their  mo- 
tion, but  one  of  its  particular  features  discloses  more. 
For  it  implies  the  past  cause  which  set  this  motion  going. 
A  distinctive  detail  of  these  spirals,  which  so  far  as  we 
know  is  shared  by  all  of  them,  are  the  two  arms  which 
leave  the  centre  from  diametrically  opposite  sides. 
This  indicates  that  the  outward  driving  force  acted 
only  in  two  places,  the  one  the  antipodes  of  the  other. 
Now  what  kind  of  force  is  capable  of  this  peculiar 
effect  ?  If  we  think  of  the  matter,  we  shall  realize  that 
tidal  action  would  produce  just  this  result.  We  see 
it  daily  in  the  case  of  the  Moon;  when  it  is  high  tide 
in  the  open  ocean  hereabouts,  it  is  high  tide  also  at  the 
opposite  end  of  the  Earth.  The  reason  is  that  the  tide- 
raising  body  pulls  the  fluid  nearest  it  more  strongly 
than  it  pulls  the  Earth  as  a  whole,  and  pulls  the  Earth 
as  a  whole  more  than  it  pulls  the  fluid  at  the  opposite 
extremity. 

Suppose,  now,  a  stranger  to  approach  a  body  in  space 
near  enough ;  it  will  inevitably  raise  tides  in  the  other's 
mass,  and  if  the  approach  be  very  close,  the  tides  will  be 
so  great  as  to  tear  the  body  in  pieces  along  the  line  due 
to  their  action ;  that  is,  parts  of  the  body  will  be  sepa- 
rated from  the  main  mass  in  two  antipodal  directions. 
This  is  precisely  what  we  see  in  the  spiral  nebula.  Nor 
is  there  any  other  action  that  we  know  of  which  would 
thus  handle  the  body.  If  it  were  to  disintegrate  under 


BIRTH    OF   A   SOLAR   SYSTEM          25 

increased  speed  of  rotation  due  to  contraction  upon  it- 
self, parts  of  its  periphery  should  be  shed  continually 
and  a  pinwheel  of  matter,  not  a  two-armed  spiral,  be 
thrown  off.  If  explosion  were  the  disintegrating  cause, 
disruption  would  occur  unsymmetrically  in  one  or  more 
directions,  not  symmetrically  as  here. 

As  the  stranger  passed  on,  his  effect  would  diminish 
until  his  attraction  no  longer  overbalanced  that  of  the 
body  for  its  disrupted  portions.  These  might  then  be 
controlled  and  forced  to  move  in  elliptic  orbits  about 
the  mass  of  which  they  had  originally  made  part. 
Thence  would  come  into  being  a  solar  system,  the  knots 
in  the  nebula  going  to  form  the  planets  that  were  to  be. 

Before  proceeding  to  what  proof  we  have  that  it 
actually  did  occur  in  this  way  we  may  pause  to  consider 
some  consequences  of  what  we  have  already  learned. 
Thus  what  brought  about  the  beginning  of  the  system 
may  also  compass  its  end.  If  one  random  encounter 
took  place  in  the  past,  a  second  is  as  likely  to  occur  in 
the  future.  Another  celestial  body  may  any  day  run 
into  the  Sun,  and  it  is  to  a  dark  body  that  we  must  look 
for  such  destruction,  because  they  are  so  much  more 
numerous  in  space. 

That  any  of  the  lucent  stars,  the  stars  commonly  so 
called,  could  collide  with  the  Sun,  or  come  near  enough 
to  amount  to  the  same  thing,  is  demonstrably  impos- 
sible for  aeons  of  years.  But  this  is  far  from  the  case 


26         THE    EVOLUTION   OF   WORLDS 

for  a  dark  star.  Such  a  body  might  well  be  within  a 
hundredth  of  the  distance  of  the  nearest  of  our  known 
neighbors,  Alpha  Centauri,  at  the  present  moment 
without  our  being  aware  of  it  at  all.  Our  senses  could 
only  be  cognizant  of  its  proximity  by  the  borrowed  light 
it  reflected  from  our  own  Sun.  Dark  in  itself,  our  own 
head-lights  alone  would  show  it  up  when  close  upon  us. 
It  would  loom  out  of  the  void  thus  suddenly  before  the 
crash. 

We  can  calculate  how  much  warning  we  should  have 
of  the  coming  catastrophe.  The  Sun  with  its  retinue 
is  speeding  through  space  at  the  rate  of  eleven  miles  a 
second  toward  a  point  near  the  bright  star  Vega.  Since 
the  tramp  would  probably  also  be  in  motion  with  a  speed 
comparable  with  our  own,  it  might  hit  us  coming  from 
any  point  in  space,  the  likelihood  depending  upon  the 
direction  and  amount  of  its  own  speed.  So  that  at  the 
present  moment  such  a  body  may  be  in  any  part  of  the 
sky.  But  the  chances  are  greatest  if  it  be  coming  from 
the  direction  toward  which  the  sun  is  travelling,  since 
it  would  then  be  approaching  us  head  on.  If  it  were 
travelling  itself  as  fast  as  the  Sun,  its  relative  speed  of 
approach  would  be  twenty-two  miles  a  second. 

The  previousness  of  the  warning  would  depend  upon 
the  stranger's  size.  The  warning  would  be  long  ac- 
cording as  the  stranger  was  large.  Let  us  assume  it 
the  mass  of  the  Sun,  a  most  probable  supposition. 


BIRTH   OF   A   SOLAR    SYSTEM          27 

Being  dark,  it  must  have  cooled  to  a  solid,  and  its  density 
therefore  be  much  greater  than  the  Sun's,  probably 
something  like  eight  times  as  great,  giving  it  a  diameter 
about  half  his  or  four  hundred  and  thirty  thousand  miles. 
Its  apparent  brightness  would  depend  both  upon  its 
distance  and  upon  its  intrinsic  brightness  or  albedo,  and 
this  last  would  itself  vary  according  to  its  distance  from 
the  Sun.  While  it  was  still  in  the  depths  of  space  and 
its  atmosphere  lay  inert,  owing  to  the  cold  there,  its 
intrinsic  brightness  might  be  that  of  the  Moon  or  Mer- 
cury. As  its  own  rotation  would  greatly  affect  the 
speed  with  which  its  sunward  side  was  warmed,  we  can 
form  no  exact  idea  of  the  law  of  its  increase  in  light. 
That  the  augmentation  would  be  great  we  see  from  the 
behavior  of  comets  as  they  approach  the  great  hearth 
of  our  solar  system.  But  we  are  not  called  upon  to 
evaluate  the  question  to  that  nicety.  We  shall  assume, 
therefore,  that  its  brilliancy  would  be  only  that  of  the 
Moon,  remembering  that  the  last  stages  of  its  fateful 
journey  would  be  much  more  resplendently  set  off. 

With  these  data  we  can  find  how  long  it  would  be 
visible  before  the  collision  occurred.  As  a  very  small 
telescopic  star  it  would  undoubtedly  escape  detection. 
It  is  not  likely  that  the  stranger  would  be  noticed  simply 
from  its  appearance  until  it  had  attained  the  eleventh 
magnitude.  It  would  then  be  one  hundred  and  forty- 
nine  astronomical  units  from  the  Sun  or  at  five  times 


28         THE   EVOLUTION   OF   WORLDS 

the  distance  of  Neptune.  But  its  detection  would  come 
about  not  through  the  eye  of  the  body,  but  through  the 
eye  of  the  mind.  Long  before  it  could  have  attracted 
man's  attention  to  itself  directly  its  effects  would  have 
betrayed  it.  Previous,  indeed,  to  its  possible  showing  in 
any  telescope  the  behavior  of  the  outer  planets  of  the 
system  would  have  revealed  its  presence.  The  far  plum- 
met of  man's  analysis  would  have  sounded  the  cause  of 
their  disturbance  and  pointed  out  the  point  from  which 
that  disturbance  came.  Celestial  mechanics  would  have 
foretold,  as  once  the  discovery  of  another  planet,  so  now 
the  end  of  the  world.  Unexplained  perturbations  in 
the  motions  of  the  planets,  the  far  tremors  of  its  coming, 
would  have  spoken  to  astronomers  as  the  first  heralding 
of  the  stranger  and  of  the  destruction  it  was  about  to 
bring.  Neptune  and  Uranus  would  begin  to  deviate 
from  their  prescribed  paths  in  a  manner  not  to  be  ac- 
counted for  except  by  the  action  of  some  new  force. 
Their  perturbations  would  resemble  those  caused  by  an 
unknown  exterior  planet,  but  with  this  difference  that  the 
period  of  the  disturbance  would  be  exactly  that  of  the  dis- 
turbed planet's  own  period  of  revolution  round  the  Sun. 
Our  exterior  sentinels  might  fail  thus  to  give  us  warn- 
ing of  the  foreign  body  because  of  being  at  the  time  in 
the  opposite  parts  of  their  orbits.  We  should  then  be 
first  apprised  of  its  coming  by  Saturn,  which  would 
give  us  less  prefatory  notice. 


BIRTH   OF   A   SOLAR   SYSTEM          29 

It  would  be  some  twenty-seven  years  from  the  time 
it  entered  the  range  of  vision  of  our  present  telescopes 
before  it  rose  to  that  of  the  unarmed  eye.  It  would  then 
have  reached  forty-nine  astronomical  units'  distance, 
or  two-thirds  as  far  again  as  Neptune.  From  here, 
however,  its  approach  would  be  more  rapid.  Humanity 
by  this  time  would  have  been  made  acquainted  with 
its  sinister  intent  from  astronomic  calculation,  and 
would  watch  its  slow  gaining  in  conspicuousness  with 
ever  growing  alarm.  During  the  next  three  years  it 
would  have  ominously  increased  to  a  first  magnitude  star, 
and  two  years  and  three  months  more  have  reached  the 
distance  of  Jupiter  and  surpassed  by  far  in  lustre 
Venus  at  her  brightest. 

Meanwhile  the  disturbance  occasioned  not  simply  in 
the  outer  planets  but  in  our  own  Earth  would  have 
become  very  alarming  indeed.  The  seasons  would  have 
been  already  greatly  changed,  and  the  year  itself  length- 
ened, and  all  these  changes  fraught  with  danger  to  every- 
thing upon  the  Earth's  face  would  momentarily  grow 
worse.  In  one  hundred  and  forty-five  days  from  the 
time  it  passed  the  distance  of  Jupiter  it  would  reach 
the  distance  of  the  Earth.  Coming  from  Vega,  it  would 
not  hit  the  Earth  or  any  of  the  outer  planets,  as  the  Sun's 
way  is  inclined  to  the  planetary  planes  by  some  sixty 
degrees,  but  the  effects  would  be  none  the  less  marked 
for  that.  Day  and  night  alone  of  our  astronomic  re- 


30         THE   EVOLUTION  OF   WORLDS 

lations  would  remain.  It  would  be  like  going  mad  and 
yet  remaining  conscious  of  the  fact.  Instead  of  follow- 
ing the  Sun  we  should  now  in  whole  or  part,  according 
to  the  direction  of  its  approach,  obey  the  stranger.  For 
nineteen  more  days  this  frightful  chaos  would  continue; 
as  like  some  comet  glorified  a  thousand  fold  the  tramp 
dropped  silently  upon  the  Sun.  Toward  the  close  of 
the  nineteenth  day  the  catastrophe  would  occur,  and 
almost  in  merciful  deliverance  from  the  already  chaotic 
cataclysm  and  the  yet  greater  horror  of  its  contempla- 
tion, we  should  know  no  more. 

Unless  the  universe  is  otherwise  articulated  than  we 
have  reason  to  suppose,  such  a  catastrophe  sometime 
seems  certain.  But  we  may  bear  ourselves  with  equa- 
nimity in  its  prospect  for  two  mitigating  details.  One  is 
that  there  is  no  sign  whatever  at  the  moment  that  any 
such  stranger  is  near.  The  unaccounted-for  errors  in  the 
planetary  theories  are  not  such  as  point  to  the  advent  of 
any  tramp.  Another  is,  that  judged  by  any  scale  of 
time  we  know,  the  chance  of  such  occurrence  is  im- 
measurably remote.  Not  only  may  each  of  us  rest 
content  in  the  thought  that  he  will  die  from  causes  of 
his  own  choosing  or  neglect,  but  the  Earth  herself  will 
cease  to  be  a  possible  abode  of  life,  and  even  the  Sun 
will  have  become  cold  and  dark  and  dead  so  long  be- 
fore that  day  arrives  that  when  the  final  shock  shall 
come,  it  will  be  quite  ready  for  another  resurrection. 


CHAPTER  II 

EVIDENCE    OF   THE    INITIAL  CATASTROPHE    IN    OUR   OWN 

CASE 

BY  quite  another  class  of  dark  bodies  than  those  we 
contemplated  in  the  last  chapter  is  the  immediate 
space  about  us  tenanted.  For  that,  too,  is  anything 
but  the  void  our  senses  give  us  to  understand.  Could 
we  rise  a  hundred  miles  above  the  Earth's  surface  we 
should  be  highly  sorry  we  came,  for  we  should  incon- 
tinently be  killed  by  flying  brickbats.  Instead  of 
masses  of  a  sunlike  size  we  should  have  to  do  with  bits 
of  matter  on  the  average  smaller  than  ourselves  but 
hardly  on  that  account  innocuous,  as  they  would  strike 
us  with  fifteen  hundred  times  the  speed  of  an  express 
train.  Only  in  one  respect  are  the  two  classes  of 
erratics  alike,  both  remain  invisible  till  they  are  upon 
us.  Even  so,  the  cause  of  their  visibility  is  different. 
The  one  is  announced  by  the  light  it  reflects,  the  other 
by  the  glow  it  gives  out  on  its  destruction.  These  last 
are  the  meteorites  or  shooting-stars.  They  are  as  well 
known  to  every  one  for  their  commonness  as,  fortunately, 
the  first  are  rare.  On  any  starlight  night  one  need  not 
tarry  long  before  one  of  these  visitants  darts  across 

31 


32         THE   EVOLUTION   OF  WORLDS 

the  sky,  a  brilliant  thread  of  fire  gone  almost  ere  it  be 
descried. 

Usually  this  is  all  of  which  one  is  made  aware. 
Silent,  ghostlike,  the  apparition  comes  and  goes,  and 
nothing  more  of  it  is  either  seen  or  heard.  But  some- 
times there  is  a  good  deal  more.  Occasionally  a  large 
ball  of  flame  shoots  through  the  air,  a  detonation  like 
distant  thunder  startles  the  ear,  and  a  luminous  train, 
persisting  for  several  seconds,  floats  slowly  away. 
Finally  if  one  be  fortunate  to  be  near,  —  but  not  too 
near,  —  one  or  more  masses  of  stone  are  seen  to  fall 
swiftly  and  bury  themselves  in  the  ground.  These  are 
meteorites :  far  wanderers  come  at  last  to  rest  in  graves 
they  have  dug  themselves. 

A  great  revolution  has  taken  place  lately  in  our  ideas 
concerning  meteorites.  Indeed,  it  was  not  so  very  long 
ago,  since  modern  man  admitted  their  astronomic 
character  at  all.  He  looked  as  askance  at  them  as  he 
did  at  fossils.  It  was  the  fall  at  Aigle,  in  Switzerland, 
April  26,  1803,  that  first  opened  men's  eyes  to  the  fact 
that  such  falls  actually  occurred.  It  is  more  than  a  nine 
days'  wonder  at  times  how  long  men,  as  well  as  puppies, 
can  remain  blind.  To  admit  that  stones  fell  from 
heaven,  however,  was  not  to  see  whence  they  came. 
Their  paternity  was  imputed  to  nearly  every  body  in  the 
sky.  They  were  at  first  supposed  to  have  been  ejected 
from  earthly  volcanic  vents,  then  from  volcanoes  in 


THE   INITIAL  CATASTROPHE  33 

the  Moon.  That  they  are  of  domestic  manufacture  is, 
however,  negatived  by  the  paths  they  severally  pursue. 
Nor  can  they  for  like  reason  have  been  ejected  from  the 
Sun. 

The  Earth  was  not  their  birthplace.  It  is  alien 
ground  in  which  they  lie  at  last  and  from  which  we 
transfer  them  to  glass  cases  in  our  museums.  This 
fact  about  their  parentage  they  tell  by  the  speed  with 
which  they  enter  our  air.  They  become  visible  100 
miles  up  and  explode  at  from  20  to  10,  and  their  speed 
has  been  found  to  be  from  10  to  40  miles  a  second, 
which  is  that  of  cosmic  bodies  moving  in  large  elliptic 
orbits  about  the  Sun,  —  a  speed  greater  than  the  Earth 
could  ever  have  imparted. 

Four  classes  of  such  small  celestial  bodies  tenant  space 
where  the  planets  move:  sporadic  shooting-stars,  me- 
teorites, meteor-streams,  and  comets.  The  discovery 
of  the  relation  of -each  of  these  to  the  solar  system  and 
then  to  each  other  forms  one  of  the  latest  chapters  of 
astronomic  history.  For  they  turn  out  to  be  generically 
one. 

It  was  long,  however,  before  this  was  perceived.  The 
first  step  was  taken  simultaneously  by  Professor  Olm- 
stead  of  Yale  and  Twining  in  1833  from  reasoning  on 
the  superb  November  meteor-shower  of  that  year. 
All  the  shooting-stars,  "thick  as  snowflakes  in  a  storm," 
had  a  common  radiant  from  which  they  seemed  to  come. 

D 


34 


THE   EVOLUTION   OF   WORLDS 


Thus  they  argued  that  the  meteors  must  all  be  travelling 
in  parallel  lines  along  an  orbit  which  the  previous  shower, 
of  1799,  showed  to  be  periodic.  This  was  the  first  rec- 
ognition of  a  meteor-swarm. 

The  next  advance  was  when  Schiaparelli,  in  1862, 
pointed  out  the  remarkable  connection  between  meteor- 
swarms  and  comets.  On  calculation  the  August  meteor- 
stream  and  the  comet  of  1862  proved  to  be  pursuing 
exactly  the  same  path.  Soon  other  instances  of  like 
association  were  discovered,  and  we  now  know  mathe- 
matically that  meteor-streams  can  be,  deductively  that 
they  must  be,  and  observationally  that  they  are,  dis- 
integrated comets.  More  than  one  comet  has  even 
been  seen  to  split. 

Then  came  the  recognition  that  comets  are  not 
visitors  from  space,  as  Sir  Isaac  Newton  and  Laplace 
supposed,  but  part  and  parcel  of  our  own  solar  system. 
Without  going  into  the  history  of  the  subject,  which 
includes  Gauss,  Schiaparelli,  and  finally  Fabry's  great 
Memoir,  much  too  little  known,  the  proof  can,  I  think, 
be  made  comprehensible  without  too  much  technique, 
thanks  to  the  fact  that  the  Sun  is  speeding  through  space 
at  the  rate  of  eleven  miles  a  second. 

Orbits  described  by  bodies  under  the  action  of  a  cen- 
tral force  are  always  conic  sections,  as  Sir  Isaac  Newton 
proved.  There  are  two  classes  of  such  curves:  those 
which  return  into  themselves,  such  as  the  circle  and 


THE   INITIAL   CATASTROPHE  35 

ellipse,  and  those  which  do  not,  the  hyperbolae.  If  a 
body  travel  in  the  first  or  closed  class  about  the  Sun,  it 
is  clearly  a  member  of  his  family;  if  in  the  second,  it  is 
a  visitor  who  bows  to  him  only  in  passing  and  never 
returns.  Which  orbit  it  shall  pursue  depends  at  a 
given  distance  solely  upon  the  speed  of  the  body;  if 
that  speed  be  one  the  Sun  can  control,  the  body  will 
move  in  an  ellipse;  if  greater,  in  an  hyperbola.  Ob- 
viously the  Sun  can  control  just  the  speed  he  can 
impart.  Now  a  comet  entering  the  system  from  with- 
out would  already  possess  a  motion  of  its  own  which, 
when  compounded  with  the  solar-acquired  speed, 
would  make  one  greater  than  the  Sun  could  master. 
Comets,  therefore,  if  visitors  from  space,  should  all 
move  in  hyperbolae.  None  for  certain  do;  and  only 
six  out  of  four  hundred  even  hint  at  it.  Comets,  then, 
are  all  members  of  the  solar  family,  excentric  ones, 
but  not  to  be  denied  recognition  of  kinship  for  such 
behavior. 

Still,  admittance  to  the  solar  family  circle  was  denied 
to  meteorites  and  shooting-stars.  Thus  Professor  Kirk- 
wood,  in  1861,  had  considered  "that  the  motions  of 
some  luminous  meteors  (or  cometoids,  as  perhaps  they 
might  be  called)  have  been  decidedly  indicative  of  an 
origin  beyond  the  limits  of  the  solar  system."  Here 
cometoid  was  an  apt  coinage,  but  when  comets  were 
later  shown  not  to  be  of  extra-solar  origin,  the  reasoning 


36         THE   EVOLUTION   OF   WORLDS 

carried  luminous  meteors  in  its  train.*  Finally  Schia- 
parelli,  in  1871,  concluded  an  able  Memoir  on  the  sub- 
ject with  the  decision  that  "  a  stellar  origin  for  meteorites 
was  the  most  likely  and  that  meteorites  were  identifiable 
with  shooting-stars/'  f  A  pregnant  remark  this,  though 
not  exactly  as  the  author  thought,  for  instead  of 
proving  both  interstellar,  as  he  intended,  both  have 
proved  to  be  solar  bound. 

It  was  Professor  Newton,  in  1889,  who  first  showed 
that  meteorites  were  pursuing,  as  a  rule,  small  elliptic 
orbits  about  the  Sun,  and  that  their  motion  was  direct. 
He,  too,  was  the  first  to  surmise  that  meteorites  are  but 
bigger  shooting-stars. 

Now,  as  to  their  connection.  Of  direct  evidence  we 
have  little.  A  few  meteors  have  been  observed  to  come 
from  the  known  radiants  of  shooting-stars.  Two  in- 
stances we  have  of  the  fall  of  meteorites  during  star 
showers.  One  in  1095,  when  the  Saxon  Chronicle  tells 
us  stars  fell  "so  thickly  that  no  man  could  count  them, 
one  of  which  struck  the  ground  and  when  a  bystander 
cast  water  upon  it  steam  was  raised  with  a  great  noise 
of  boiling."  The  second  case  was  the  fall  of  a  sider- 
ite,  eight  pounds'  worth  of  nickel-iron,  at  Mazapil 
during  the  Andromede  shower  of  1885,  which  was  by 

*  "  Mem.  del  Reale  Inst.  Lombardo,"  Vol.  XII.  Ill  della 
serie  III. 

f  Quoted  in  "  Luminous  Meteors,"  Committee's  Report  for 
1870-1871,  p.  48. 


THE   INITIAL   CATASTROPHE 


37 


many  supposed  to  be  a  part  of  the  lost  Biela  comet. 
It  contained  graphite  enough  to  pencil  its  own  history, 
but  unfortunately  could  not  write.  The  direction  from 


THE  RADIANT  OF  A  METEORIC  SHOWER,  SHOWING  ALSO  THE  PATHS  OF  THREE 
METEORS  WHICH  DO  NOT  BELONG  TO  THIS  SHOWER  —  AFTER  DENNING. 

which  it  came  was  not  recorded,  and  so  the  connection 
between  it  and  the  comet  not  made  out. 

If  our  direct  knowledge  is  thus  scanty,  reasoning 
affords  surer  ground  for  belief.  For  at  this  point  there 
steps  in  a  bit  of  news  about  the  family  relations  of 
shooting-stars  from  a  source  hardly  to  have  been  antici- 
pated. Indeed,  it  arose  from  the  thought  to  examine  a 
qualitative  statement  in  Young's  "  Astronomy  "  quanti- 
tatively. Mathematics  is  simply  precise  reasoning, 


38         THE   EVOLUTION   OF  WORLDS 

applied  usually  to  the  discovery  that  a  pet  theory  will 
not  work.  But  sometimes  it  presents  one  with  an 
unexpected  find.  This  is  what  it  did  here. 

It  is  an  interesting  fact  of  observation  that  more 
meteors  are  visible  at  six  o'clock  in  the  morning  than  at 
six  o'clock  at  night  in  the  proportion  of  3  to  i.  This 


SUNRISE 


METEORS 
Diagram  explaining  their  proportionate  visibility. 

denotes  true  paths. 

"       apparent  paths. 

.  ••       Earth's  path. 


seeming  preference  for  early  rising  is  due  to  no  matuti- 
nality  on  the  part  of  the  meteors,  but  to  the  matin  aspect 
then  presented  by  the  Earth  combined  with  its  orbital 
motion  round  the  Sun.  For  at  six  in  the  morning  the 
observer  stands  on  the  advancing  side  of  the  Earth, 
at  the  bow  of  the  airship ;  at  six  at  night  he  is  at  the 
stern.  He,  therefore,  runs  into  the  meteors  at  sunrise 
and  slips  away  from  them  at  sunset.  He  is  pelted  in  the 
morning  in  consequence.  Just  as  a  pedestrian  facing 
a  storm  gets  wetter  in  front  than  behind. 

So  far  the  books.     Now  let  us  examine  this  quanti- 
tatively according  to  the  direction  in  which  the  meteors 


THE   INITIAL  CATASTROPHE  39 

themselves  may  be  moving  before  the  encounter.  Sup- 
pose, in  the  first  place,  that  they  were  travelling  in  every 
possible  direction,  with  the  average  velocity  of  the  most 
erratic  members  of  the  family,  the  great  comets.  On 
this  supposition  calculation  shows  that  we  ought  to 
meet  5.8  times  as  many  at  six  in  the  morning  as  at  six 
at  night.  If  their  orbits  were  smaller  than  this,  say, 
something  like  those  of  the  asteroids,  we  should  find  7.6 
to  i  for  the  ratio. 

Suppose,  however,  that  they  were  all  travelling  in  the 
same  sense  as  the  Earth,  direct  as  it  is  called  in  con- 
tradistinction to  retrograde,  and  let  us  calculate  what 
proportion  in  that  case  we  should  meet  at  the  two 
hours  respectively.  It  turns  out  to  be  2.4  to  I  for  the 
parabolic  ones,  3.3  to  I  for  the  smaller  orbited,  or  al- 
most precisely  what  observation  shows  to  be  the  case.1* 
Here,  then,  a  bit  of  abstract  reasoning  has  apprized  us  of 
a  most  interesting  family  fact;  to  wit,  that  the  great 
majority  of  shooting-stars  are  travelling  in  the  same 
orderly  sense  as  ourselves.  Furthermore,  as  some  must 
be  moving  in  smaller  orbits  than  the  mean,  others  must 
be  journeying  in  greater;  or,  in  other  words,  shooting- 
stars  are  scattered  throughout  the  system.  In  short, 
these  little  bodies  are  tiny  planets  themselves,  as  truly 
planets  as  the  asteroids,  —  asteroids  of  a  general  instead 
of  a  localized  habit. 

*  Numerals  refer  to  notes  at  end  of  book. 


40         THE   EVOLUTION   OF  WORLDS 

Thus  meteorites  and  shooting-stars  are  kin,  and  from 
the  fact  that  they  are  pursuing  orbits  not  very  unlike 
our  own  we  get  our  initial  hint  of  a  community  of  origin. 
Indeed,  they  are  the  little  bricks  out  of  which  the  whole 
structure  of  our  solar  system  was  built  up.  What  we 
encounter  to-day  are  the  left-over  fragments  of  what 
once  was,  the  fraction  that  has  not  as  yet  been  swept 
up  by  the  larger  bodies.  And  this  is  why  these  latter- 
day  survivors  move,  as  a  rule,  direct.  To  run  counter 
to  the  consensus  of  trend  is  to  be  subjected  to  greater 
chance  of  extermination.  Those  that  did  so  have 
already  been  weeded  out. 

From  the  behavior  of  meteorites  we  proceed  to  scan 
their  appearance.  And  here  we  notice  some  further 
telltale  facts  about  them.  Their  conduct  informed  us 
of  their  relationship,  their  character  bespeaks  their 
parentage. 

Most  meteorites  are  stones,  but  one  or  two  per  cent 
are  nearly  pure  iron  mixed  with  nickel.  When  picked 
up,  they  are  usually  covered  with  a  glossy  thin  black 
crust.  This  overcoat  they  have  put  on  in  coming 
through  our  air.  Air-begotten,  too,  are  the  holes  with 
which  many  of  them  are  pitted.  For  entering  our  at- 
mosphere with  their  speed  in  space  is  equivalent  to 
immersing  them  suddenly  in  a  blowpipe  flame  of  sev- 
eral thousand  degrees  Fahrenheit.  Thus  their  sur- 
face is  burnt  and  fused  to  a  cinder.  Yet  in  spite  of 


THE   INITIAL   CATASTROPHE 


being  warm  to  the  touch  their  hearts  are  still  cosmically 

cold.     The    Dhurmsala    meteorite    falling    into    moist 

earth  was  found  an  hour  afterwards  coated  with  frost. 

Agassiz  likened  it  to  the 

Chinese    culinary    chef 

d'ceuvre  "fried  ice."     It 

is  the  cold  of  space,  200° 

or  more  Centigrade  be- 
low zero,  that  they  bear 

within,    proof    of    their 

cosmic  habitat. 

That  they  are  bits  of 

a    once    larger    mass    is 

evident    on    their    face. 

Their  shape  shows  that 

they  are  not  wholes  but 
parts,  while  their  con- 
stitution bespeaks  them 
anything  but  elementary. 
Diagnosis  of  it  yields 
perhaps  their  most  interesting  bit  of  news.  For  it  shows 
their  origin.  Their  autopsy  proves  them  to  contain 
thirty  known  elements,  and  not  one  that  is  new.  The 
list  includes  all  the  substances  most  common  on  the 
Earth's  surface,  which  is  suggestive;  but,  what  is  still 
more  instructive,  these  are  combined  into  minerals 
which  largely  differ  from  those  with  which  we  are  super- 


THE  MART  IRON. 
(Proc.  Wash.  Acad.  ofSci.  vol.  II.  plate  VI.) 


42         THE   EVOLUTION   OF  WORLDS 

ficially  familiar.  Professor  Newton,  whose  specialty 
they  were,  has  said :  "  In  general  they  show  no  resem- 
blance in  their  mechanical  or  mineralogical  structure 
to  the  granitic  and  surface  rocks  of  the  Earth.  One 


SECTION  OF  METEORITE  SHOWING  WIDMANNSTATTIAN  LINES. 
(Field  Columbian  Museum,  Chicago.) 

condition  was  certainly  necessary  in  their  formation, 
viz.  the  absence  of  free  oxygen  and  of  enough  water  to 
oxidize  the  iron."  Thus  they  are  not  of  the  Earth 
earthy;  nor  yet,  poor  little  waifs,  of  the  upper  crust  of 
any  other  body. 

In  them  prove  to  be  occluded  gases,  which  can  be  got 
out  by  heating  in  the  laboratory,  and  which  must  have 
got  in  when  the  meteorites  were  still  subjected  to  great 
heat  and  pressure.  For  only  thus  could  these  gases 
have  been  absorbed.  Both  such  heat  and  such  pressure 
accuse  some  great  solid  body  as  origin  of  this  flotsam 
of  the  sky.  Fragments  now,  they  owe  to  its  disruption 


THE   INITIAL   CATASTROPHE 


43 


their  present  separate  state.  This  parent  mass  must 
have  been  much  larger  and  more  massive  than  the 
Earth,  as  the  great  amount  of  occluded  hydrogen, 
sometimes  one- 
third  the  vol- 
ume at  500°  C., 
of  the  meteorite 
seems  to  testify. 
The  two 
classes  of  mete- 
orites, the  stone  <\.;  > 
and  the  iron, 
show  this  fur- 
ther by  the  very 
differences  they  exhibit  between  themselves.  For  both 
the  amount  and  the  proportions  of  the  occluded  gases 
in  the  two  prove  to  be  quite  distinct.  In  the  stones  the 
quantity  of  gas  is  greater  and  the  composition  is  di- 
verse. In  the  stones  carbonic  acid  gas  is  common, 
carbon  monoxide  rare;  in  the  irons  the  ratio  is  just  the 
other  way.  Thus  Wright  found  in  nine  specimens  of 
the  iron  meteorites:  — 


METEORITE,  TOLUCA. 
(Field  Columbian  Museum,  Chicago.) 


C02  CO  H 

u-596       32.4%  54.1 
in  ten  of  stone :  - 

CO2  CO  H 

60.  i  %        3.4%  32.0 


CH4 

oo  %  of  the  total ; 

CH4 

2.1  % 


44         THE   EVOLUTION   OF   WORLDS 

The  stones  are  much  lighter  than  the  iron,  their 
specific  gravities  being  as  3  to  7  or  8  for  the  metallic. 
The  stones,  therefore,  came  from  a  more  superficial 
layer  of  the  body  torn  apart  than  the  iron,  and  the 
composition  of  their  occluded  gases  bears  this  out. 
Those  in  the  stones  are  such  as  we  may  conceive  ab- 
sorbed nearer  the  surface,  those  in  the  iron  from  re- 
gions deeper  down. 

Here,  then,  the  meteorites  tell  us  of  another,  an 
earlier,  stage  of  our  solar  system's  history,  one  that 
mounts  back  to  before  even  the  nebula  arose  to  which 
we  owe  our  birth.  For  the  large  body  to  whose  dis- 
memberment the  meteorites  were  due  can  have  been 
no  other  than  the  one  whose  cataclysmic  shattering 
produced  that  very  nebula  which  was  for  us  the  origin 
of  things.  The  meteorites,  by  continuing  unchanged, 
link  the  present  to  that  far-off  past.  And  they  tell  us, 
too,  that  this  body  must  have  been  dark.  For  solid, 
they  inform  us,  it  was,  and  solidity  in  a  heavenly  body 
means  deficiency  of  light. 

That  such  corroborative  testimony  to  a  cataclysmic 
origin  is  forthcoming  in  the  sky  we  shall  see  by  turning 
again  to  the  spiral  nebulae. 

Of  the  two  classes  of  nebulae  which  we  contemplated 
in  the  last  chapter,  the  amorphous  and  the  structural, 
there  is  more  to  be  said  than  we  touched  on  then. 

Not  only  in  look  are  the  two  quite  unlike,  but  the 


THE   INITIAL   CATASTROPHE 


45 


spectroscope  shows  that  the  difference  in  appearance 
is  associated  with  dissimilarity  of  character.  For  the 
spectrum  of  the  amorphous  proves  to  consist  of  a  few 
bright  lines,  due  to  hydrogen  and  nebulium  chiefly,  in 


NEBULA  ijl  V.  14  CYGNI  —  AFTER  ROBERTS. 

the  green,  whence  the  name  green  nebulae.  That  of 
the  spirals,  on  the  other  hand,  is  continuous,  and  there- 
fore white.  The  great  nebula  in  Andromeda  was  one 
of  the  first  in  which  this  was  recognized ;  and  the  per- 
ception was  pregnant,  for  no  nebula  defies  resolution 
more  determinedly  than  it.  We  may,  therefore,  infer 


46         THE   EVOLUTION   OF  WORLDS 

that  it  is  not  made  up  of  stars,  certainly  big  enough  for 
us  to  see.  On  the  other  hand,  from  the  fact  that  its 
spectrum  is  continuous  it  must  be  solid  or  liquid. 
Young  pointed  out  that  this  did  not  follow,  because 
a  gas  under  great  pressure  also  gives  a  continuous 


NEBULA  N.  G.  C.  1499  PERSEI  —  AFTER  ROBERTS. 

spectrum.  But  he  forgot  that  here  no  such  pressure 
could  exist.  A  nebula  of  compressed  gas  could  not 
have  an  irregular  form  and  would  have,  in  the  case  of 
the  Andromeda  nebula,  a  mass  so  enormous  as  to  pre- 
clude supposition.  Continuity  of  spectrum  here  means 
discontinuity  of  mass.  The  spectral  solidity  of  the 
nebula  speaks  of  a  status  quo  ante,  not  of  a  condition 
of  condensation  now  going  on. 


THE   INITIAL   CATASTROPHE  47 

Advanced    spectroscopic    means    reveals    that    the 
spectra  of  these  "  white "   nebulae  are  not  simply  con- 


NEBULA  N.  G.  C.  6960  IN  CYGNUS  —  AFTER  RITCHEY. 

tinuous.  Thus  that  of  the  Andromeda  nebula  shows 
very  faint  dark  lines  crossing  it,  apparently  accordant 
with  those  of  the  solar  spectrum  and  faint  bright  ones 
falling  near  and  probably  coincident  with  those  of  the 


48         THE   EVOLUTION   OF  WORLDS 

Wolf-Rayet    stars,   due  to  hydrogen,   helium,   and    so 
forth.     These  later  observations  make  practically  cer- 


NEBULA  M.  51  CANUM  VENATICORUM  —  AFTER  RITCHEY. 

tain  what  earlier  ones  permitted  us  just  now  only  to  in- 
fer: that  it  is  not  composed  of  stars,  but  of  something 
subtler  still;  to  wit,  of  meteorites.  The  reasoning  is 
interesting,  as  showing  that  if  one  have  hold  of  a  true 
idea,  the  stars  in  their  courses  fight  for  him. 

Although  Lockyer  has  long  been  of  opinion  that  the 


THE   INITIAL   CATASTROPHE  49 

nebulae  are  composed  of  meteorites,  the  present 
argument  differs  from  his.  The  way  in  which  their 
spectra  establish  their  constitution  may  be  outlined  as 
follows:  the  white  nebulae  are  from  their  structure 
evidently  in  process  of  evolution,  and  if  they  are  in  stable 
motion,  as  we  suppose  them  to  be,  their  parts  are  mov- 
ing round  their  common  centre  of  gravity.  As  the 
white  nebulae  resist  resolution  as  obstinately  as  the  green, 
these  parts  must  be  not  only  solid  but  comminuted 
(composed  of  small  particles).  Now  this  would  be  the 
case  were  they  flocks  of  meteorites  such  as  we  have  seen 
composed  our  own  system  once  upon  a  time.  Though 
all  are  travelling  round  the  centre  of  gravity  of  the 
flock,  each  is  pursuing  its  own  orbit  slightly  different 
from,  and  intersecting  those  of,  its  neighbors.  Colli- 
sions between  the  meteors  must  therefore  constantly 
occur,  and  the  question  is,  are  these  shocks  sufficient  to 
cause  light.  Let  us  take  our  own  system  and  consider 
two  meteorites  at  our  distance  from  the  Sun,  travelling 
in  the  same  sense,  the  one  in  an  ellipse,  the  other  in  a 
circle,  with  a  major  axis  five  per  cent  greater  and  meet- 
ing the  other  at  aphelion.  This  would  be  no  improper 
jostle  for  such  heavenly  bodies.  If  we  calculate  the 
speeds  of  both  and  deduct  the  elliptic  from  the  circular, 
we  shall  have  the  relative  speed  of  collision.  It  proves 
to  be  a  half  a  mile  a  second  or  30  times  the  speed  of  an 
express  train.  As  such  a  train  brought  up  suddenly 


50         THE   EVOLUTION  OF  WORLDS 

against  a  stone  wall  would  certainly  elicit  sparks,  we  see 
that  a  speed  30  times  as  great,  whose  energy  is  900  times 
greater,  is  quite  competent  to  a  shock  sufficient  to 
make  us  see  stars  en  masse.  But,  indeed,  there  must  be 
collisions  much  more  violent  than  this;  both  because 
the  central  mass  is  often  much  greater  and  because  the 
orbits  differ  much  more,  and  the  effect  would  increase 
as  the  square  of  the  speed.  The  heat  thus  generated 
would  cause  the  meteorites  to  glow,  and  at  the  same 
time  raise  the  temperature  of  the  gases  in  and  about 
them.  Furthermore,  the  light  would  come  to  us  through 
other  non-affected  portions  of  gas  between  us  and  the 
scene  of  the  collision.  Thus  all  three  peculiarities  of 
the  spectra  stand  explained  :  we  have  a  continuous  back- 
ground of  light  due  to  heated  solid  meteorites,  the  bright 
lines  of  glowing  gases,  and  dark  lines  due  to  other  gases 
not  ignited,  lying  in  our  line  of  sight. 

In  addition  we  should  perceive  another  result.  Col- 
lisions would  be  both  more  numerous  and  more  pro- 
nounced toward  the  centre  of  the  nebula,  for  it  must 
speedily  grow  denser  toward  its  core  owing  to  the  fall- 
ing in  of  meteorites,  in  consequence  of  shock.  Being 
denser  in  the  centre,  the  particles  would  there  be  thicker 
and  be  travelling  at  greater  speed.  The  nebulae,  there- 
fore, should  be  brightest  at  their  centres,  which  is  ac- 
cordant with  observation. 

Thus  from  having  offered  themselves  exemplars  of  the 


THE   INITIAL   CATASTROPHE  51 

way  in  which  our  own  system  came  into  being,  the  white 
nebulae  assert  their  present  constitution  to  be  that  from 
which  we  know  our  system  sprang. 

Another  suggestive  fact  about  the  present  members 
of  our  solar  system  which  has  something  to  say  about 
a  past  collision  is  the  densities  of  the  different  planets. 
The  average  density  of  the  four  inner  planets,  Mars, 
the  Earth,  Venus,  and  Mercury  is  nearly  four  times 
that  of  the  four  outer  ones  Neptune,  Uranus,  Saturn, 
and  Jupiter.2  The  discrepancy  is  striking  and  cannot 
be  explained  by  size,  as  the  smallest  are  the  most  mas- 
sive, and  if  all  were  primally  of  like  constitution,  should 
be  the  least  compressed.  Nor  can  it  be  explained  simply 
by  greater  heat  tending  to  expand  them,  for  Neptune 
and  Uranus  show  no  signs  of  being  very  hot.  The 
minor  differences  between  members  of  each  group  are 
probably  explicable  in  part  by  these  two  factors,  mass 
and  heat,  but  the  great  gulf  between  the  two  groups 
cannot  so  be  spanned.  We  are  then  driven  to  the  sup- 
position that  the  materials  composing  the  outer  ones 
were  originally  lighter.  Now  this  is  precisely  what 
should  happen  had  all  eight  been  formed  by  disruption 
of  a  previous  body.  For  its  cuticle  would  be  its  least 
dense  portion,  and  on  disruption  would  travel  farthest 
away,  not  because  of  being  lighter,  but  because  of 
being  on  the  outside.  Parts  coming  from  deeper  down 
would  remain  near,  and  be  denser  intrinsically. 


52         THE   EVOLUTION   OF  WORLDS 

What  the  present  densities  of  the  planets  enable  us 
to  infer  of  the  cataclysm  from  which  they  came,  a  re- 
markable set  of  spectrograms  taken  not  long  ago  by 
Dr.  V.  M.  Slipher,  at  Flagstaff,  seems  to  confirm. 

The  spectrograms  in  question  were  made  possible  by 
his  production  of  a  new  kind  of  plate.  His  object  was 
to  obtain  one  which  should  combine  sufficient  speed 
with  great  photographic  extension  of  the  spectrum  into 
the  red.  For  it  is  in  the  red  end  that  the  absorption 
lines  due  to  the  planets'  atmospheres  chiefly  lie.  With 
the  plates  heretofore  used  it  was  impossible  to  go  much 
beyond  the  yellow,  the  C  line  marking  the  Ultima 
Thule  of  attent.  Not  only  was  it  advisable  to  get  more 
particularity  in  the  parts  previously  explored,  but  it  was 
imperative  to  go  beyond  into  parts  as  yet  unknown. 
After  several  attempts  he  succeeded,  the  plates  when 
exposed  showing  the  spectra  beyond  even  the  A  band. 
Of  their  wealth  of  depiction  it  is  only  necessary  to  say 
that  in  the  spectrum  of  Neptune  130  lines  and  bands  can 
easily  be  counted  between  the  wave-lengths  4600  pp., 
7600  pp.  Of  these  31  belong  to  the  planet,  which 
compares  with  6  found  by  Huggins,  10  by  Vogel,  and 
9  by  Keeler  in  the  part  of  its  spectrum  they  were  able 
to  obtain. 

The  result  was  a  revelation.  The  plates  exposed 
a  host  of  lines  never  previously  seen ;  lines  that  do  not 
appear  in  the  spectrum  of  the  Sun,  nor  yet  in  the  added 


EC 
Ul 


o 
o 
Z 


-fc- 


Ld 


m 

O 


THE   INITIAL   CATASTROPHE  53 

spectrum  of  the  atmosphere  of  the  Earth,  but  are  due 
to  the  planets'  own  envelopes.  But  this  was  only  the 
starting-point  of  their  disclosures.  When  in  this  man- 
ner he  had  taken  the  color  signatures  of  Jupiter,  Saturn, 
Uranus,  and  Neptune,  an  orderly  sequence  in  their 
respective  absorption  bands  stood  strikingly  confessed. 
In  other  words,  their  atmospheres  proved  not  only 
peculiar  to  themselves  and  unlike  what  we  have  on 
Earth,  but  progressively  so  according  to  a  definite  law. 
That  law  was  distance  from  the  Sun.  When  the  spectra 
were  arranged  vertically  in  ordered  orbital  relation 
outward  from  the  Sun,  with  that  of  the  lunar  for  com- 
parison on  top,  a  surprising  progression  showed  down 
the  column  in  the  strange  bands,  an  increase  in  number 
and  a  progressive  deepening  in  tint.  The  lunar,  of 
course,  gives  us  the  Sun  and  our  own  air.  All  else  must 
therefore  be  of  the  individual  planet's  own.  Beginning, 
then,  with  Jupiter,  we  note,  besides  the  reenforcement  of 
what  we  know  to  be  the  great  water-vapor  bands  <  0,'  sev- 
eral new  ones,  which  show  still  darker  in  the  spectrum  of 
Saturn.  The  strongest  of  these  is  apparently  not  identi- 
fiable with  a  band  in  the  spectra  of  Mira  Ceti  in  spite  of 
falling  near  it.  Passing  on  to  Uranus,  we  perceive  these 
bands  still  more  accentuated,  and  with  them  others, 
some  strangers,  some  solar  lines  enhanced.  Thus  the 
hydrogen  lines  stand  out  as  in  the  Sirian  stars.  All 
deepen  in  Neptune,  while  further  newcomers  appear. 


54         THE   EVOLUTION   OF   WORLDS 

Thus  we  are  sure  that  free  hydrogen  exists  in  large 
quantities  in  the  atmospheres  of  the  two  outermost 
planets  and  most  so  in  the  one  farthest  off.  Helium, 
too,  apparently  is  there,  and  other  gases  which  in  part 
may  be  those  of  long-period  stars,  decadent  suns,  in 
part  substances  we  do  not  know. 

From  the  fact  that  these  bands  are  not  present  in  ths 
Sun  and  apparently  in  no  type  of  stars,  we  may  perhaps 
infer  that  the  substances  occasioning  them  are  not  ele- 
ments but  compounds  to  us  unknown.  And  from  the 
fact  that  free  hydrogen  exists  there  alongside  of  them, 
and  apparently  helium,  too,  we  may  further  conclude 
that  they  are  of  a  lighter  order  than  can  be  retained  by 
the  Earth. 

But  now,  we  may  ask,  why  should  these  lighter  gases 
be  found  where  they  are  ?  It  cannot  be  in  consequence 
simply  of  the  kinetic  theory  of  gases  from  which  a  corol- 
lary shows  that  the  heaviest  bodies  would  retain  their 
gases  longest,  because  the  strange  gases  are  not  appor- 
tioned according  to  the  sizes  of  their  hosts.  Jupiter, 
by  all  odds  the  biggest  in  mass,  has  the  least,  and 
Saturn,  the  next  weightiest,  the  next  in  amount.  Nor 
can  title  to  such  gaseous  ownership  be  lodged  in  the 
planet's  present  state.  For  though  Jupiter  is  the 
hottest  and  Saturn  the  next  so,  the  increased  mass  more 
than  makes  up  in  restraint  what  increased  temperature 
adds  in  molecular  volatility  —  as  we  perceive  in  the 
cases  of  the  Sun  and  Earth. 


THE    INITIAL   CATASTROPHE  55 

No;  their  envelopes  are  increasingly  strange  because 
their  internal  constituents  are  different,  and  as  hydro- 
gen is  most  abundant  in  Neptune,  the  lightest  of  all 
the  gases,  it  is  inferable  that  this  planet's  material  is 
lighter.  As  distance  from  the  Sun  determines  their 
atmospheric  clothing,  so  distance  decides  upon  their 
bodies,  too.  It  was  all  a  case  of  primogeniture.  The 
light  strange  matter  that  constitutes  them  was  so  be- 
cause it  came  from  the  outer  part  of  the  dismembered 
parent  orb.  Neptune  the  outermost,  Uranus  the  next, 
then  Saturn  and  Jupiter  came  in  that  order  from  the 
several  successive  layers  of  the  pristine  body,  while 
the  inner  planets  came  from  parts  of  it  deeper  down. 
The  major  planets  were  of  the  skin  of  the  dismembered 
body,  we  of  its  lower  flesh. 

Very  interesting  the  study  of  these  curious  spectral 
lines  from  the  outer  planets  for  themselves  alone; 
even  more  so  for  what  one  would  hardly  have  imagined : 
that  they  should  actually  tell  us  something  of  the  genesis 
of  our  whole  solar  system.  They  corroborate  in  so  far 
what  the  meteorites  have  to  say. 

That  the  meteorites  are  solid  and,  except  for  their 
experiences  in  coming  through  our  air,  bear  no  marks  of 
external  heat,  is  a  fact  which  is  itself  significant.  It 
seems  to  hint  not  at  a  crash  as  their  occasioning  but 
at  disruptive  tidal  strains.  The  parent  body  appears 
to  have  been  torn  apart  without  much  development  of 


56         THE   EVOLUTION  OF   WORLDS 

heat.  Perhaps,  then,  we  had  no  gloriously  pyrotechnic 
birth,  but  a  more  modest  coming  into  existence.  But. 
about  this  we  must  ourselves  modestly  be  content  to 
remain  for  the  present  in  the  dark. 

Not  the  least  important  feature  of  the  theory  I  have 
thus  outlined  is  that  it  finishes  out  the  round  of  evolu- 
tion. It  becomes  a  conception  sapiens  in  se  ipso  totus, 
teres  atque  rotundus.  To  frame  a  theory  that  carries 
one  back  into  the  past,  to  leave  one  there  hung  up  in 
heaven,  is  for  inconclusiveness  as  bad  as  the  ancient 
fabulous  support  of  the  world,  which  Atlas  carried 
standing  on  an  elephant  upheld  by  a  tortoise.  What 
supported  the  tortoise  we  were  not  told.  So  here,  if 
meteorites  were  our  occasioning,  we  must  account  for  the 
meteorites,  starting  from  our  present  state.  This  the 
present  presentation  does. 

Thus  do  the  stones  that  fall  from  the  sky  inform  us 
of  two  historic  events  in  our  solar  system's  career.  They 
tell  us  first  and  directly  of  a  nebula  made  up  of  them,  out 
of  which  the  several  planets  were  by  agglomeration 
formed  and  of  which  material  they  are  the  last  un- 
gathered  remains.  And  then  they  speak  to  us  more 
remotely  but  with  no  less  certainty  of  a  time  antedating 
that  nebula  itself,  a  time  when  the  nebula's  constitu- 
ents still  lay  enfolded  in  the  womb  of  a  former  Sun. 

Man's  interest  in  them  hitherto  has  been,  as  with 
other  things,  chiefly  proprietary.  Greed  of  them  has 


THE   INITIAL    CATASTROPHE  57 

grown  so  keen  that  legal  questions  have  been  raised  of 
the  ownership  of  their  rinding,  and  our  courts  have 
solemnly  declared  them  not  "wild  game"  but  "real 
estate, "  and  as  such  belonging  to  the  owner  of  the  land 
on  which  they  fall. 

But  to  the  scientific  eye  their  estate  is  something 
more  than  "real,"  for  theirs  is  the  oldest  real  estate  in 
the  solar  system.  They  were  what  they  are  now  when 
the  Earth  we  pride  ourselves  in  owning  was  but  a 
molten  mass. 

So  that  when  in  future  you  see  these  strange  stones 
in  rows  upon  a  museum's  shelves,  regard  them  not  as 
rarities,  in  which  each  museum  strives  to  outdo  its 
neighbors  by  the  quantity  it  can  possess,  but  as  rosetta 
stones  telling  us  of  an  epoch  in  cosmic  history  long  since 
passed  away  —  of  which  they  alone  hold  the  key. 
Look  at  them  as  the  literary  do  their  books,  for  that 
which  they  contain,  not  as  the  bibliophile  to  whom  a 
misprint  copy  outvalues  a  corrected  one  and  by  whom 
"uncuts"  are  the  most  prized  of  all. 


CHAPTER   III 

THE    INNER    PLANETS 

WHEN  we  recall  that  the  Ptolemaic  system  of  the 
universe  was  once  taught  side  by  side  with  the 
Copernican  at  Harvard  and  at  Yale,  we  are  impressed, 
not  so  much  with  the  age  of  our  universities,  as  with 
the  youth  of  modern  astronomy  and  with  the  extraordi- 
nary vitality  of  old  ideas.  That  the  Ptolemaic  system 
in  its  fundamental  principle  was  antiquated  at  the 
start,  the  older  Greeks  having  had  juster  concep- 
tions, does  not  lessen  our  wonder  at  its  tenacity.  But 
the  fact  helps  us  to  understand  why  so  much  fossil 
error  holds  its  ground  in  many  astronomic  text-books 
to-day.  That  stale  intellectual  bread  is  deemed  better 
for  the  digestion  of  the  young,  is  one  reason  why  it 
often  seems  to  them  so  dry. 

Before  entering  upon  the  problem  of  the  genesis 
and  career  of  a  world,  it  is  essential  to  have  acquaint- 
ance with  the  data  upon  which  our  deductions  are 
to  rest.  To  set  forth,  therefore,  what  is  known  of  the 
several  planets  of  our  solar  system,  is  a  necessary 
preliminary  to  any  understanding  of  how  they  came 
to  be  or  whither  they  are  tending;  and  as  our  knowl- 

58 


THE   INNER    PLANETS 


59 


edge  has  been  vitally  affected  by  modern  discoveries 
about  them,  it  is  imperative  that  this  exposition  of 
the  facts  should  be  as  near  as  possible  abreast  of  the 
research  itself.  I  shall,  therefore,  give  the  reader 


ORBITS  OF  THE  INNER  PLANETS. 

in  this  chapter  a  bird's-eye  view  of  the  present  state 
of  planetary  astronomy,  which  he  will  find  almost  a 
different  part  of  speech  from  what  it  was  thirty  years 
ago.  It  is  not  so  much  in  our  knowledge  of  their 
paths  as  of  their  persons  that  our  acquaintance  with 


60         THE   EVOLUTION   OF   WORLDS 

the  planets  has  been  improved.  And  this  knowledge 
it  is  which  has  made  possible  our  study  of  their  evo- 
lution as  worlds. 

Could  we  get  a  cosmic  view  of  the  solar  system  by 
leaving  the  world  we  live  on  for  some  suitable  vantage- 
point  in  space,  two  attributes  of  it  would  impose  them- 
selves upon  us  —  the  general  symmetry  of  the  whole, 
and  the  impressively  graded  proportions  of  its  par- 
ticular parts. 

Round  a  great  central  globular  mass,  the  Sun,  far 
exceeding  in  size  any  of  his  attendants,  circle  a  series 
of  bodies  at  distances  from  him  quite  vast,  compared 
with  their  dimensions.  These,  his  principal  planets, 
are  in  their  turn  centres  to  satellite  systems  of  like 
character,  but  on  a  correspondingly  reduced  scale. 
All  of  them  travel  substantially  in  one  plane,  a  fact 
giving  the  system  thus  seen  in  its  entirety  a  remark- 
ably level  appearance,  as  of  an  ideal  surface  passing 
through  the  centre  of  the  Sun.  Departing  somewhat 
from  this  general  uniformity  in  their  directions  of 
motion,  and  also  deviating  more  from  circularity  in 
their  paths,  some  much  smaller  bodies,  a  certain  dis- 
tance out,  dart  now  up  now  down  across  it  at  different 
angles  and  from  all  the  points  of  the  compass,  agree- 
ing with  the  others  only  in  having  the  centre  of  the 
Sun  their  seemingly  never  attained  goal  of  endeavor. 
These  bodies  are  the  asteroids.  Surrounding  the 


THE   INNER   PLANETS  61 

whole,  and  even  penetrating  within  its  orderly  pre- 
cincts, a  third  class  would  be  visible  which  might  be 
described  for  size  as  cosmic  dust,  and  for  display  as 
heavenly  pyrotechnics.  Coming  from  all  parts  of 
space  indifferently  they  would  seem  to  seek  the  Sun 
in  almost  straight  lines,  bow  to  him  in  circuit,  and 
then  depart  whence  they  came.  For  in  such  long  ellip- 
ses do  they  journey  that  these  seem  to  be  parabolas. 
These  visitants  are  the  comets  and  their  associates  the 
meteor  streams. 

Although  for  purposes  of  discrimination  we  have 
labelled  the  several  classes  apart,  an  essential  fact  about 
the  whole  company  is  to  be  noted:  that  no  hard 
and  fast  line  can  be  drawn  separating  the  several 
constituents  from  one  another.  In  size  the  members 
of  the  one  class  merge  insensibly  into  the  other.  Some 
of  the  planets  are  hardly  larger  than  some  of  the  satel- 
lites; some  of  the  satellites  than  some  of  the  asteroids; 
some  of  the  asteroids  than  comets  and  shooting  stars. 
In  path,  too,  we  find  every  gradation  from  almost 
perfect  circularity  like  the  orbits  of  lo  and  Europa 
to  the  very  threshold  of  where  one  step  more  would 
cease  to  leave  the  body  a  member  of  the  Sun's  family 
by  turning  its  ellipse  into  an  hyperbola.  Finally,  in 
inclination  we  have  every  angle  of  departure  from  or- 
thodox platitude  to  unconforming  uprightness.  This 
point,  that  heavenly  bodies,  like  terrestrial  ones,  show 


62         THE   EVOLUTION   OF   WORLDS 

all  possible  grades  of  indistinction,  is  kin  to  that  spe- 
cific generalization  by  which  Darwin  revolutionized 
zoology  a  generation  ago.  It  is  as  fundamental  to 
planets  as  to  plants.  For  it  shows  that  the  whole  solar 
system  is  evolutionarily  one. 

A  second  point  to  be  noticed  in  passing  is  that 
undue  inclination  and  excessive  eccentricity  go  to- 
gether. The  bodies  that  have  their  paths  least  circu- 
lar have  them,  as  a  rule,  the  most  atilt.  And  with 
these  two  qualities  goes  lack  of  size.  It  is  the  smallest 
bodies  that  deviate  most  from  the  general  consensus 
of  the  system.  With  so  much  by  way  of  generic 
preface,  the  pregnancy  of  which  will  become  apparent 
as  we  proceed,  we  come  now  to  particular  considera- 
tion of  its  members  in  turn. 

Nearest  to  the  Sun  of  all  the  planets  comes  Mer- 
cury. So  close  is  he  to  that  luminary,  and  so  far  within 
the  orbit  of  the  earth,  that  he  is  not  a  very  common 
object  to  the  unaided  eye.  Copernicus  is  said  never 
to  have  seen  him,  owing,  doubtless,  to  the  mists  of  the 
Vistula.  By  knowing  when  to  look,  however,  he 
may  be  seen  for  a  few  days  early  in  the  spring  in  the 
west  after  sunset,  or  before  sunrise  in  the  east  in  au- 
tumn. He  is  then  conspicuous,  being  about  as  bright 
as  Capella,  for  which  star  or  Arcturus  he  is  easily 
mistaken  by  one  not  familiar  with  the  constellations. 

His  mean  distance  from  the  Sun  is  thirty-six  million 


THE   INNER   PLANETS  63 

miles,  but  so  eccentric  is  his  orbit,  the  most  so  of  any  of 
the  principal  planets,  that  he  is  at  times  half  as  far 
off  again  as  at  others.  Even  his  orbital  behavior  is 
the  least  understood  of  any  in  the  solar  system.  His 
orbit  swings  round  at  a  rate  which  so  far  has  defied 
analysis.  It  may  be  a  case  of  reflected  perturbation, 
one,  that  is,  of  which  the  indirect  effect  from  another 
body  becomes  more  perceptible  than  would  be  the 
direct  effect  on  the  body  itself.  As  yet  it  baffles 
geometers. 

As  to  his  person,  our  ignorance  until  lately  was 
profound.  It  is  only  recently  that  such  fundamental 
facts  about  him  as  his  size,  his  mass,  and  his  density 
have  been  reached  with  any  approach  to  precision. 
This  was  because  he  so  closely  hugs  the  Sun  that 
observations  upon  his  full,  or  nearly  full,  disk  had 
never  been  attempted.  When  I  say  that  his  volume 
was  not  known  to  within  a  third  of  its  amount,  his 
mass  not  closer  than  one-half,  while  his  received 
density  was  nearly  double  what  we  now  have  reason 
to  suppose  the  fact,  some  idea  of  the  depth  of  our  nes- 
cience may  be  imagined.  This,  of  course,  did  not  pre- 
vent text-books  from  confidently  misinstructing  youth, 
or  Nautical  Almanacs  from  misguiding  computers  with 
figures  that  thus  almost  achieved  immortality,  so  long 
had  they  passed  current  in  spite  of  lacking  that  per- 
fection which  is  usually  assigned  as  its  warrant. 


64 


THE   EVOLUTION   OF   WORLDS 


Schiaparelli  first  put  astronomy  on  the  right  track. 
By  attempting  daylight  observations  of  the  planet,  not 


SULLA  ROTAZIONE  DI  MERCURIC  —  Di  G.  V.  SCHIAPARELLI. 

toward  night,  but  actually  at  midday,  he  made  some 
remarkable  discoveries,  and  though  he  did  not  detect 
the  hitherto  erroneous  values  of  the  volume,  the  mass, 
or  the  density,  his  method  of  observation  paved  the 
way  for  their  ascertainment.  What  he  sought,  and 
found,  was  evidence  of  markings  upon  the  disk  by 


THE   INNER   PLANETS  65 

which  the  planet's  time  of  rotation  might  be  deter- 
mined. Up  to  then,  Schroeter's  value  of  about  twenty- 
four  hours  had  been  accepted,  on  very  slender  evidence 
indeed,  and  passed  into  all  the  books.  But  when  the 
planet  came  to  be  observed  by  noon,  very  definite 
markings  stood  out  on  its  face,  which  showed  its  rota- 
tion to  take  place,  not  in  twenty-four  hours,  but  in  eighty- 
eight  days.  By  a  persistence  equal  to  his  able  choice 
of  observing  time,  he  established  this  beyond  dispute. 
He  proved  the  revolutionizing  fact  that  Mercury's  pe- 
riods of  rotation  and  of  revolution  were  the  same. 

He  detected,  too,  the  evidence  in  the  position  of  the 
markings  of  the  planet's  great  libratory  swing  due  to 
the  eccentricity  of  its  orbit,  a  result  as  remarkable  as 
a  feat  of  observation  as  it  was  conclusive  as  a  proof. 

If  Schiaparelli  had  never  done  any  other  astronom- 
ical work,  this  study  of  Mercury  would  have  placed 
him  as  the  first  observer  of  his  day.  For  the  observa- 
tions are  so  difficult  that  the  planet  not  only  baffled 
all  his  predecessors,  but  has  foiled  many  since  who 
are  credited  with  being  observers  of  eminence. 

In  1896  the  study  of  Mercury  was  taken  up  at  the 
Lowell  Observatory  in  Arizona  along  the  same  lines 
that  had  proved  so  successful  with  Schiaparelli,  but 
without  using  his  observations  as  guide.  Indeed,  his 
papers  had  not  then  been  read  there.  The  two  con- 
clusions were,  therefore,  independent  of  one  another. 


66         THE   EVOLUTION   OF  WORLDS 

The  outcome  was  a  complete  corroboration  and  an 
extension  of  Schiaparelli's  work.  We  shall  begin 
with  the  consideration  of  the  most  fundamental  point. 
In  the  clear  and  steady  air  of  Flagstaff,  permitting  of 
measurement  of  his  disk  up  to  within  a  few  degrees 
of  the  Sun,  Mercury  was  found  to  be  much  larger 
than  previously  thought. 

Instead  of  a  diameter  of  three  thousand  miles  he 
proved  to  have  one  of  thirty-four  hundred,  making 
his  volume  nearly  half  as  large  again  as  had  been 
credited  him.  These  measures  bore  intrinsic  evi- 
dence of  their  trustworthiness  in  an  interesting  man- 
ner, and  at  the  same  time  produced  internal  testi- 
mony that  accounted  for  the  smallness  of  previous 
determinations.  Measures  heretofore  had  been  made, 
usually  if  not  invariably,  either  when  the  planet 
transited  the  Sun  or  when  it  exhibited  a  pronounced 
phase.  Now  in  both  these  cases  the  planet  looks 
smaller  than  it  is.  In  the  first  case  this  is  due  to 
irradiation,  the  surrounding  disk  of  the  Sun  encroach- 
ing both  to  the  eye  and  to  the  camera  upon  the  sil- 
houette of  Mercury.  And  this  inevitable  effect  had 
not  been  allowed  for  in  the  measures.  In  the  second 
case  the  horns  of  the  planet  never  seem  to  extend 
quite  to  their  true  position.  This  was  rendered  evi- 
dent by  the  Flagstaff  series  of  measures,  which  began 
when  the  planet  was  a  half-moon  and  continued  till  it 


THE    INNER   PLANETS  67 

was  almost  full.  As  it  did  so,  the  values  for  the  diam- 
eter steadily  increased,  even  after  irradiation  was 
allowed  for,  although  this  against  the  brilliant  back- 
ground of  the  noonday  sky  must  have  been  exceed- 
ing small,  and  tended  in  part  to  be  diminished  as  the 
planet  attained  the  full,  because  of  its  consequent  near- 
ing  of  the  Sun.  The  measures  thus  explained  them- 
selves and  vouched  for  their  own  accuracy.* 

Then  came  a  curious  bit  of  unexpected  proof  to 
corroborate  them.  In  his  "  Astronomical  Constants,"  f 
published  but  a  short  time  before,  Newcomb  had 
detected  a  systematic  error  in  the  right  ascensions  of 
Mercury  which  he  was  not  able  to  explain.  By  dili- 
gent mousing  that  eminent  computer  had  discovered 
that  Mercury  was  registered  by  observers  too  far  from 
the  Sun  on  whichever  side  of  him  it  happened  to  be, 
and  in  proportion  roughly  not  to  its  distance  off  but 
to  the  phase  the  planet  exhibited.  When  the  disk  was 
a  crescent  the  discrepancy  between  observation  and 
theory  was  large,  and  thence  decreased  as  the  planet 
passed  to  the  full.  He  suspected  the  cause,  and  would 
have  found  it  had  he  not  considered  the  diametral 
measures  of  the  planet  too  well  assured  to  permit  of 
doubt.  As  it  was,  he  neglected  a  factor  which  has 

*  New  Observations  of  the  Planet  Mercury,  Memoirs  Amer. 
Acad.  1897.  Vol.  XII,  No.  4. 

f  "  Astronomical  Constants,"  1895,  PP-  67,  68. 


68         THE   EVOLUTION   OF  WORLDS 

vitiated  almost  all  the  observations  made  on  the  planets 
up  to  within  a  few  years,  the  correction  for  irradia- 
tion. This  was  the  case  here.  The  received  meas- 
ures, beginning  with  Bradley  and  ending  with  Todd, 
had  almost  without  exception  been  made  in  transit, 
and,  as  no  regard  had  been  paid  to  the  contracting 
effect  of  irradiation,  had  been  invalidated  in  conse- 
quence. The  new  method  supplied  almost  exactly  the 
amount  needed  to  explain  the  right  ascensions,  a  sec- 
ond of  arc,  and  in  precise  accordance  with  the  place 
which  the  discrepancy  demanded. 

About  the  mass  there  has  been,  and  still  is,  great 
uncertainty.  This  is  because  it  can  only  be  found 
from  the  perturbing  effect  it  has  on  Venus,  the  Earth, 
or  Encke's  comet.  Modern  determinations,  however, 
are  smaller  than  the  older  ones;  thus  Backlund  in 
1894  got  from  the  effect  on  Encke's  comet  only  one- 
half  the  mass  that  Encke  had,  fifty-three  years  before. 
Probably  the  most  reliable  information  comes  from 
Venus,  which  Tisserand  found  to  give  for  Mercury 
710oooo  of  the  mass  of  the  Sun,  or  ^-  of  the  mass  of 
the  Earth.  If  we  take  TooTFFoo'  as  t^ie  nearest  round 
number,  we  find  the  planet's  density  to  be  0.66  that 
of  the  Earth. 

The  same  observations  that  disclosed  at  Flagstaff 
the  planet's  size  revealed  a  set  of  markings  on  his  face 
so  definite  as  to  make  the  rotation  period  unmistakable. 


THE    INNER    PLANETS 


69 


It  takes  place,  as  Schiaparelli  found,  in  eighty-eight 
days,  or  the  time  of  the  planet's  revolution  round  the 
Sun.  The  markings  disclosed  the  fact,  as  Schiaparelli 
had  also  discovered,  in  a  most  interesting  manner,  for 


MAP    CF    MERCURY 


the  ellipticity  of  the  planet's  orbit  stood  reflected  in 
the  swing  of  the  markings  across  the  face  of  the  disk, 
a  definiteness  in  the  proof  of  a  really  surprising  kind. 
What  this  means  we  shall  see  in  a  subsequent  chap- 
ter when  we  take  up  the  mechanical  problem  of  the 
tides.  Another  result  that  issued  from  the  positions 
of  the  markings  was  the  determination  of  the  planet's 
pole.  Except  for  the  libration  above  noticed,  the 


7o         THE   EVOLUTION   OF   WORLDS 

markings  kept  an  invariable  longitudinal  position 
upon  the  illuminated  disk,  showing  that  the  planet 
turned  always  the  same  face  to  the  Sun;  but  latitudi- 
nally  a  difference  was  noticeable  between  their  place  in 
October-November,  1896,  and  in  February-March, 
1897,  the  latter  being  4°  farther  north.  Now  this 
is  just  what  the  orbital  position  should  have  caused, 
if  the  pole  stood  vertically  to  it.  Thus  a  difference 
of  4°  from  perpendicularity  should  have  been  dis- 
cernible, had  it  existed, —  a  very  small  amount  in 
such  a  determination.  We  may,  therefore,  conclude 
that  the  axis  stands  plumb  to  the  orbit,  and  this  is 
what  theory  demands. 

The  state  of  things  this  introduces  to  us  upon  that 
other  world  is  to  our  ideas  exceeding  strange.  It  is 
not  so  much  the  slowness  of  the  diurnal  spin,  eighty- 
eight  times  as  long  as  our  own,  which  is  surprising,  as 
the  fact  that  this  makes  its  day  infinite  in  length. 
Two  antipodal  hemispheres  divide  the  planet,  the  one 
of  which  frizzles  under  eternal  sun,  the  other  freezes 
amid  everlasting  night.  The  Sun  does  not,  indeed, 
stand  stock-still  in  the  sky,  but  nods  like  some  huge 
pendulum  to  and  fro  along  a  parallel  of  latitude.  In 
consequence  of  libration  the  two  great  domains  of 
day  and  night  are  sundered  by  a  strip  of  debatable 
ground  23  J°  in  breadth  on  either  side,  upon  which  the 
Sun  alternately  rises  and  sets.  Here  there  is  a  true 


THE   INNER    PLANETS  71 

day,  eighty-eight  of  our  days  in  length  from  one  sun- 
rise to  the  next.  But  its  day  and  night  are  not  ap- 
portioned alike.  The  eastern  strip  has  its  daylight 
briefer  than  its  starlight  hours;  the  western  has  them 
longer.  Nor  are  different  portions  of  the  strips  simi- 
larly circumstanced  in  their  sunward  regard.  Only 
the  edge  next  perpetual  day  has  anything  approach- 
ing an  equal  distribution  of  sunlight  and  shade.  The 
farther  one  just  peeps  at  the  Sun  for  a  moment  every 
eighty-eight  days,  and  then  sinks  back  again  into  ob- 
scurity. 

The  transition  from  day  to  night  is  equally  instan- 
taneous and  profound.  For  little  or  no  twilight  here 
prolongs  the  light;  since  the  air,  if  there  be  any  at  all, 
is  too  thin  to  bend  it  to  service  round  the  edge  to 
illuminate  the  night.  When  the  libratory  Sun  sets, 
darkness  like  a  mantle  falls  swiftly  over  the  face  of 
the  ground.  No  evidence  of  atmosphere  has  ever 
been  perceived,  and  theory  informs  that  it  should  be 
nearly,  if  not  wholly,  absent. 

In  consequence  of  the  rigid  uprightness  of  the 
planet's  axis,  seasons  do  not  exist.  Their  nearest 
simulacrum  comes  from  the  seeming  dilatation  of  the 
Sun  during  half  the  year,  and  its  apparent  contraction 
during  the  other  half.  It  expands  so  much  between 
its  January  and  its  July  as  to  receive  more  heat  in  the 
ratio  of  nine  to  four.  A  seasonless,  dayless,  and  almost 


72         THE   EVOLUTION   OF   WORLDS 

yearless  planet,  it  is  better  to  look  at  than  to  look 
from;  but  its  study  opens  our  eyes  to  the  great  diver- 
sity which  even  one  of  our  nearest  neighbors  exhibits 
from  what  we  take  as  matters  of  course  on  Earth. 

That  what  we  take  offhand  to  be  purely  astronomic 
phenomena  should  turn  out  to  be  so  essentially  of  the 
particular  world,  worldly,  clarifies  vision  of  what 
these  really  are,  and  how  dependent  on  and  interwoven 
with  everyday  life  astronomy  is.  Or,  we  may  consider 
it  turned  about  and  realize  how  purely  astronomic 
relations,  such  abstract  mechanical  matters  as  rota- 
tions and  revolutions,  result  in  completely  changing 
the  very  face  and  character  of  the  globe  concerned. 
Mercury  to-day  stares  forever  at  the  Sun.  The 
markings  we  see  have  stereotyped  this  stare  to  its  in- 
evitable result.  For  they  seem  to  mark  a  globe  sun- 
cracked.  At  such  a  condition  the  curious  crisscross 
of  dark,  irregular  lines  certainly  hints,  accentuated 
and  perfected  as  it  is  by  a  bounding  curve  where  the 
mean  sunward  side  terminates  to  the  enclosing  them 
as  by  the  carapace  of  a  tortoise.  Though  they  can- 
not probably  be  actual  cracks,  however  much  they 
may  resemble  such,  yet  they  may  well  owe  their  exist- 
ence to  that  fundamental  cause. 

In  cdlor  the  planet  is  ghastly  white;  of  that  wan 
hue  that  suggests  a  body  from  which  all  life  has  fled. 
Far  whiter  than  Venus  in  point  of  fact,  the  rosy  tint 


THE   INNER    PLANETS  73 

with  which  it  sparkles  in  the  sunset  glow  is  all  bor- 
rowed of  the  dying  day  and  vanishes  when  the  planet 
is  looked  at  in  the  uncompromising  light  of  noon. 
Seen  close  together  once  at  Flagstaff  it  was  possible 
directly  to  compare  the  two;  when  Mercury,  although 
lit  by  the  Sun  two  and  a  half  times  as  brilliantly  as 
Venus,  was,  surface  for  surface,  more  than  twice  as 
faint.  Miiller  has  found  its  intrinsic  brightness  about 
that  of  our  Moon,  which  in  some  respects  it  resembles, 
though  it  apparently  departs  widely  from  any  simi- 
larity in  others.  The  bleached  bones  of  a  world;  that 
is  what  Mercury  seems  to  be. 

Venus  comes  next  in  order  outward  from  the  Sun. 
To  us  her  incomparable  beauty  is  partly  the  result  of 
propinquity:  nearness  to  ourselves  and  nearness  to 
the  Sun.  Relatively  so  close  is  she  to  both  that  she 
does  not  need  the  Sun's  withdrawal  to  appear,  but 
may  nearly  always  be  seen  in  the  daytime  in  clear  air 
if  one  knows  where  to  look  for  her.  Situate  about 
seven-tenths  of  our  own  distance  from  our  common 
giver  of  light  and  heat,  she  gets  about  double  the 
amount  that  falls  to  our  lot,  so  that  her  surface  is  pro- 
portionately brilliantly  illuminated.  Being  also  rela- 
tively near  us,  she  displays  a  correspondingly  large 
surface. 

But  though  part  of  her  lustre  is  due  to  her  position, 
a  part  is  her  own.  Direct  visual  observation,  as  we 


74         THE   EVOLUTION   OF   WORLDS 

remarked  above,  shows  her  intrinsic  brightness  to  be 
more  than  five  times  that  of  Mercury,  square  mile  to 
square  mile  of  surface  for  the  two.  Now  this  has 
been  determined  very  carefully  photometrically  by 
Miiller  at  Potsdam.  The  result  of  his  inquiry  was  to 
indicate  that  Mercury  shines  with  0.17  of  absolute 
reflection,  Venus  with  0.92.  So  high  a  value  has 
seemed  to  many  astronomers  impossible,  because  so 
far  surpassing  that  which  has  tacitly  been  taken  as 
the  ne  plus  ultra  of  planetary  brightness,  that  of  cloud, 
0.72. 

Now,  one  of  the  direct  outcomes  of  the  study  of 
Venus  at  the  Lowell  Observatory  was  an  explanation 
of  this  seemingly  incredible  phenomenon.  When  the 
planet  came  to  be  critically  examined  there  under 
conditions  of  seeing  which  permitted  discovery,  mark- 
ings very  faint,  but  nevertheless  assurable,  stood  pre- 
sented on  the  planet's  face.  These  markings,  of  which 
we  shall  have  more  to  say  in  a  moment,  had  this  of 
pertinency  to  our  present  point,  that  they  kept  an  in- 
variable position  to  one  another.  They  thus  betrayed 
themselves  to  be  surface  features.  Furthermore,  their 
dimness  was  as  invariable  an  attribute  of  them  as  their 
place.  They  were  not  obscured  on  some  occasions 
and  revealed  at  others,  but  stayed,  so  far  as  one  might 
judge,  permanently  the  same.  They  were  thus  neither 
clouds  themselves  nor  subject  to  the  caprice  of  cloud. 


THE   INNER    PLANETS  75 

The  old  idea  that  Venus  was  a  cloud-wrapped  planet 
and  owed  her  splendor  to  this  envelope,  vanished  liter- 
ally into  thin  air. 

It  is  precisely  because  she  is  not  cloud-covered  that 
her  lustre  is  so  great.  She  "clothes  herself  with  light 
as  with  a  garment"  by  a  physical  process  of  some 
interest.  As  becomes  the  Mother  of  the  Loves,  this  is 
gauze  of  the  most  attenuated  character,  and  yet  a 
wonderful  heightener  of  effect.  For  it  consists  solely 
of  the  atmosphere  that  compasses  her  about.  It  is 
well  known  that  a  substance  when  comminuted  re- 
flects much  more  light  than  when  condensed  into  a 
solid  state.  Now  an  atmosphere  is  itself  such  a  com- 
minuted affair,  and,  furthermore,  holds  in  suspension  a 
variety  of  dust.  This  would  particularly  be  the  case 
with  the  atmosphere  of  Venus,  as  we  shall  have  reason 
to  see  when  we  consider  the  conditions  upon  that 
planet  made  evident  by  study  of  its  surface  markings. 
To  her  atmosphere,  then,  she  owes  four-fifths  or  more 
of  her  brilliancy.  And  this  stands  corroborated  by 
the  low  albedo  of  both  Mercury  and  the  Moon,  which 
have  no  atmosphere,  and  by  the  intermediate  lustre  of 
Mars,  which  has  some,  but  little.* 

The  rotation  time  of  Venus,  the  determination,  that 
is,  of  the  planet's  day,  is  one  of  the  fundamental  astro- 
nomical acquisitions  of  recent  years.  For  upon  it 

*  Astr.  Nach.  No.  3406.     Monthly  Notices  R.  A.  S.,  March,  1897. 


76         THE   EVOLUTION   OF   WORLDS 

turns  our  whole  knowledge  of  the  planet's  physical 
condition.  More  than  this,  it  adds  something  which 
must  be  reckoned  with  in  the  framing  of  any  cos- 
mogony. It  is  not  a  question  of  academic  accuracy 
merely,  of  a  little  more  or  a  little  less  in  actual 
duration,  but  one  which  carries  in  its  train  a  com- 
pletely new  outlook  on  Venus  and  sheds  a  valuable 
sidelight  upon  the  history  of  our  whole  planetary 
system. 

Unconsciously  influenced,  one  is  inclined  to  think, 
by  terrestrial  analogies,  astronomers  for  more  than  a 
couple  of  centuries,  ever  since  the  time  of  the  first 
Cassini  in  1666,  deemed  the  day  of  Venus  to  be  just 
under  twenty-four  hours  in  length.  So  well  attested 
was  its  determination,  and  so  precisely  figured  to  the 
minute,  that  it  imposed  itself  upon  text-books  which 
stated  it  as  an  acquired  fact  down  to  the  last  second. 
Nevertheless,  Schiaparelli  was  not  so  sure,  and  pro- 
ceeded to  look  into  the  matter.  He  first  looked  for 
himself,  and  then  looked  up  all  -the  old  observations. 
His  chief  observational  departure  was  observing  by 
day  as  near  to  noon  as  possible;  because  then  the 
planet  was  highest,  to  say  nothing  of  the  taking  off 
from  its  glare  by  the  more  brilliant  sky.  From  certain 
dark  markings  around  two  bright  spots  near  the 
southern  cusp,  of  one  of  which  spots  the  detection 
dates  from  the  time  of  Schroeter,  and  from  a  long, 


THE   INNER    PLANETS  77 

dark  streak  stretching  thence  well  down  the  disk,  he 
convinced  himself  that  no  such  period  as  twenty-four 
hours  could  possibly  be  correct,  inasmuch  as  whenever 
he  looked,  the  markings  were  always  there.  His  notes 
read,  "Same  appearance  as  yesterday/*  day  after  day, 
until  he  would  really  have  saved  ink  and  penmanship 
had  he  had  the  phrase  cut  into  a  die  and  stamped. 
He  concluded  that  the  rotation  was  at  least  six  months 
long,  and  was  probably  synchronous  with  the  planet's 
time  of  revolution.  This  was  in  1889.  In  1895  he 
became  still  more  sure,  and  showed  how  the  older 
observations  were  really  compatible  with  what  he  had 
found. 

In  1896  the  subject  was  taken  up  at  Flagstaff.  Very 
soon  it  became  evident  there  that  markings  existed 
on  the  disk,  most  noticeable  as  fingerlike  streaks 
pointing  in  from  the  terminator,  faint  but  unmistakable 
from  the  identity  of  their  successive  presentation. 
Schroeter's  projection  near  the  south  cusp  was  also 
clearly  discernible  as  well  as  two  others,  one  in  mid- 
terminator,  one  near  the  northern  cusp.  Schiaparelli's 
dark  markings  also  came  out,  developing  into  a  sort 
of  collar  round  the  southern  pole.  Other  spots  and 
streaks  also  were  discernible,  and  all  proved  permanent 
in  place.  By  watching  them  assiduously  it  was  pos- 
sible to  note  that  no  change  in  position  occurred  in 
them,  first  through  an  interval  of  five  hours,  then 


78         THE   EVOLUTION   OF  WORLDS 

through  one  of  days,  then  of  weeks.     Care  was  taken 
to    guard    against    illusion.    -It    thus    became    evident 


VENUS.    OCTOBER,  i896-MARCH,  1897  —  DRAWINGS  BY  DR.  LOWELL. 

that  they  bore  always  the  same  relation  to  the  illumi- 
nated  portion   of   the    disk.      This   illuminated    part, 


THE    INNER   PLANETS  79 

then,  never  changed.  In  other  words,  the  planet 
turned  always  the  same  face  to  the  Sun.  The  fact 
lay  beyond  a  doubt,  though  of  course  not  beyond  a 
doubter.* 

The  years  that  have  passed  since  these  observations 
were  made  have  brought  corroboration  of  them.     Sev- 


VENUS.    APRIL  12,  1909,  311  26M-4H  22M  —  BY  DR.  LOWELL. 

eral  observers  at  Flagstaff  have  seen  and  drawn  them 
and  added  discoveries  of  their  own,  among  whom 
are  especially  to  be  mentioned,  of  the  observatory 
staff:  Miss  Leonard,  Dr.  Slipher,  and  Mr.  E.  C. 
Slipher.f 

In  character  these  markings  were  peculiar  and  dis- 
tinctive.    In  addition  to  some  of  more  ordinary  char- 

*  Monthly  notices  R.  A.  S.,  March,  1897. 
f  Lowell  Observatory  Bulletin  6. 


8o         THE   EVOLUTION   OF   WORLDS 

acter  were  a  set  of  spokelike  streaks  which  started 
from  the  planet's  periphery  and  ran  inwards  to  a 
point  not  very  distant  from  the  centre.  The  spokes 
started  well-defined  and  broad  at  the  edge,  dwindling 
and  growing  fainter  as  they  proceeded,  requiring  the 
best  of  definition  for  their  following  to  their  central 
hub. 

The  peculiar  symmetry  thus  displayed,  a  symmetry 
associated  with  the  planet's  sunrise  and  sunset  line, 
or,  strictly  speaking,  what  would  be  such  did  the  Sun 
for  Venus  ever  rise  or  set,  would  seem  inexplicable, 
except  for  that  very  association.  When  we  reflect, 
however,  upon  what  this  means,  a  very  potent  cause 
for  them  becomes  apparent,  so  potent  that  surprise  is 
turned  into  appreciation  that  nothing  else  could  well 
exist.  That  Venus  turns  on  her  axis  in  the  same 
time  that  she  revolves  about  the  Sun,  in  consequence 
of  which  she  turns  always  the  same  face  to  him,  must 
cause  a  state  of  things  of  which  we  can  form  but  faint 
conception,  from  any  earthly  analogy.  One  face 
baked  for  countless  aeons,  and  still  baking,  backed  by 
one  chilled  by  everlasting  night,  while  both  are  still 
surrounded  by  air,  must  produce  indraughts  from 
the  cold  to  the  hot  side  of  tremendous  power.  A 
funnel-like  rise  must  take  place  in  the  centre  of  the 
illuminated  hemisphere,  and  the  partial  vacuum  thus 
formed  would  be  filled  by  air  drawn  from  its  periphery, 


THE   INNER    PLANETS 


81 


which,    in    its    turn,    would    draw   from    the    regions 
of  the  night   side.     Such  winds  would  sweep  the  sur- 


Showing  convec- 
tion currents  in  the 
planet's  atmosphere. 


II 

Showing  shift  in 
central  barometric 
depression  due  to 
rotation  of  the  planet 
affecting  the  winds. 


VENUS. 


face  as  they  entered,  becoming  less  superficial  as  they 
advanced,  and  the  marks  of  their  inrush  might  well 


82         THE   EVOLUTION   OF  WORLDS 

be  discernible  even  at  the  distance  we  are  off.  Deltas 
of  such  inroad  would  thus  seam  the  bounding  circle  of 
light  and  shade. 

Another  result  of  the  aerial  circulation  would  be 
the  removal  of  all  moisture  from  the  sunward  face, 
and  its  depositing  in  the  form  of  ice  upon  the  night 
one.  For  the  heated  air  would  be  able  to  carry  much 
water  in  suspension,  which,  on  cooling,  after  it  had 
reached  the  dark  hemisphere  would  unload  it  there. 
In  the  low  temperature  there  prevailing,  this  moisture 
would  all  be  frozen,  and  so  largely  estopped  from  re- 
turn. This  process  continuing  for  ages  would  finally 
deplete  one  side  of  all  its  water  to  heap  it  up  in  the 
form  of  ice  upon  the  other. 

Now  it  is  not  a  little  odd  that  a  phenomenon  has 
been  observed  upon  Venus  which  seems  to  display 
just  this  state  of  things.  Many  observers  have  noted 
an  ashen  light  on  the  dark  side  of  her  disk.  Some 
have  tried  to  account  for  it  as  Earth  shine,  the  same 
earth-reflected  light  that  makes  dimly  visible  the  old 
moon  in  the  new  moon's  arms.  But  the  Earth  is  too 
far  away  from  Venus  to  permit  of  any  such  effect; 
nor  is  there  any  other  body  that  could  thus  relieve  its 
night.  But  if  the  night  hemisphere  of  Venus  be  one 
vast  polar  sheet,  we  have  there  a  substance  able  to 
mirror  the  stars  to  a  ghostlike  gleam  which  might  be 
discernible  even  from  our  distant  post. 


THE   INNER    PLANETS  83 

Thus  when  we  reason  upon  them  we  see  that  the 
peculiar  markings  of  the  planet  lose  their  oddity,  be- 
coming the  very  pattern  and  prototype  of  what  we 
should  expect  to  view.  Interpreted,  they  present  us 
the  picture  of  a  plight  more  pitiable  even  than  that 
of  Mercury.  For  the  nearly  perfect  circularity  of 
Venus'  orbit  prevents  even  that  slight  change  from 
everlasting  sameness  which  the  libration  of  Mercury's 
affords.  To  Venus  the  Sun  stands  substantially  stock- 
still  in  the  sky,  —  a  fact  which  must  prove  highly  reas- 
suring to  Ptolemaic  astronomers  there,  if  there  be  any 
still  surviving  from  her  past.  No  day,  no  seasons, 
practically  no  year,  diversifies  existence  or  records 
the  flight  of  time.  Monotony  eternalized,  —  such  is 
Venus'  lot. 

What  visual  observations  have  thus  discovered  of 
the  rotation  time  of  Venus,  with  all  that  follows  from 
it,  the  spectroscope  at  Flagstaff  has  confirmed.  At 
Dr.  Slipher's  hands,  spectrograms  of  the  planet  have 
told  the  same  tale  as  the  markings.  It  was  with  spe- 
cial reference  to  this  point  that  the  spectrograph  there 
was  constructed,  and  the  first  object  to  which  it  was 
directed  was  Venus.* 

The  planet's  rotation  time  was  to  be  investigated 
by  means  of  the  motion  it  brought  about  in  the  line  of 
sight.  Visual  observation,  telescopically,  reveals  motion 
*  Lowell  Observatory  Bulletin  No.  3. 


84         THE   EVOLUTION   OF   WORLDS 

thwart-wise  by  the  displacement  it  produces  in  the  field 
of  view;  spectroscopic  observation  discloses  motion  to 
or  from  the  observer  by  the  shift  it  causes  in  the  spectral 
lines  due  to  a  stretching  or  shortening  of  their  wave- 
lengths. 

The  spectroscope  is  an  instrument  for  analyzing 
.  light.  Ordinary  light  consists  of  light  of  various 
wave-lengths.  By  means  of  a  prism  or  grating  these 
are  dispersed  into  a  colored  ribbon  or  band,  the  longer 
waves  lying  at  the  red  end  of  the  spectrum,  as  the  ribbon 
is  called,  the  shorter  at  the  violet.  Now  the  spectro- 
scope is  primarily  such  a  prism  or  grating  placed 
between  the  image  and  the  observer,  by  means  of  which 
a  series  of  colored  images  of  the  object  are  produced. 
In  order  that  these  may  not  overlap  and  so  confuse 
one  another,  the  light  is  allowed  to  enter  the  prism  only 
through  a  narrow  slit  placed  across  the  telescopic  image 
of  the  object  to  be  examined.  Thus  successive  images 
of  what  is  contained  by  the  slit  are  presented  arranged 
according  to  their  wave-lengths.  In  practice  the  rays 
of  light  from  the  slit  enter  a  small  telescope  called 
the  collimator,  and  are  there  rendered  parallel,  in  which 
condition  they  fall  upon  the  prism.  This  spreads 
them  out  into  the  spectrum  and  another  small  tele- 
scope focusses  them,  each  according  to  its  kind,  into  a 
spectral  image  band  which  may  then  be  viewed  by 
the  eye  or  caught  upon  a  photographic  plate. 


THE    INNER    PLANETS  85 

Now,  if  an  object  be  coming  toward  the  observer, 
emitting  or  reflecting  light  as  it  does  so,  each  wave- 
length of  its  spectrum  will  be  shortened  in  proportion 
to  the  relative  speed  of  its  approach  as  compared  with 
the  speed  of  light,  because  each  new  wave  is  given  out 
by  so  much  nearer  the  observer  and  in  reflection  the 
body  may  also  meet  it.  Reversely  it  will  be  length- 
ened if  the  object  be  receding  from  the  observer  or 
he  from  it.  This  would  change  the  color  of  the  object 
were  it  not  that  while  each  hue  moves  into  the  place 
of  the  next,  like  the  guests  at  Alice's  tea-party  in 
Wonderland,  some  red  rays  pass  off  the  visible  spec- 
trum, but  new  violet  rays  come  up  from  the  infra- 
violet  and  the  spectrum  is  as  complete  as  before. 
Fortunately,  however,  in  all  spectra  are  gaps  where 
individual  wave-lengths  are  absorbed  or  omitted,  and 
these,  the  lines  in  the  spectrum,  tell  the  tale  of  shift. 
Now  if  a  body  be  rotating,  one  side  of  it  will  be  ap- 
proaching the  observer,  while  the  opposite  side  is 
receding  from  him,  and  if  the  slit  be  placed  perpen- 
dicular to  the  axis  about  which  the  spin  takes  place, 
each  spectral  line  will  appear  not  straight  across  the 
spectrum  of  the  object,  but  skewed,  the  approaching 
side  being  tilted  to  the  violet  end,  the  receding  side 
to  the  red. 

This  was  to  be  the  procedure  adopted  for  the  rotation 
of  Venus.  By  placing  the  slit  parallel  to  the  ecliptic, 


86         THE   EVOLUTION   OF   WORLDS 

or,  more  properly,  to  the  orbit  of  Venus,  which  is  prac- 
tically the  same  thing,  it  found  itself  along  what  we 
have  reason  to  suppose  the  equator  of  the  planet. 
Even  a  considerable  error  on  this  point  would  make 
little  difference  in  the  rotational  result.  In  order  that 
there  might  be  no  question  of  illusion  or  personal 
bias,  photographs  instead  of  eye  observations  of  the 
spectrum  were  made.  For  reference  and  check  side 
by  side  with  that  of  Venus  were  taken  on  either  hand 
the  spectra  of  iron,  made  by  sparking  a  tube  containing 
the  vapor  of  that  metal.  The  vapor,  of  course,  had  no 
motion  with  regard  to  the  observer,  and  therefore  its 
spectral  lines  could  have  no  tilt,  but  must  represent 
motional  verticality. 

Dr.  Slipher  chose  his  time  astutely.  He  selected 
the  occasion  when  Venus  was  passing  through  supe- 
rior conjunction,  or  the  point  in  her  orbit  as  regards  us 
directly  beyond  the  sun.  At  first  sight  this  might 
seem  to  be  the  worst  as  well  as  the  most  impracticable 
of  epochs,  inasmuch  as  the  planet  is  then  not  only  at 
her  farthest  from  the  Earth,  but  in  a  line  with  the  Sun, 
and  so  drowned  in  his  glare.  But  in  point  of  fact 
any  tilt  of  the  spectral  lines  is  then,  owing  to  phase, 
twice  what  it  is  at  elongation,  and  exceeds  still  more 
what  it  is  when  Venus  has  her  greatest  lustre.3  In  his 
purpose  he  was  abetted  by  the  Flagstaff  air,  which 
enabled  the  planet  to  be  spectrographed  much  nearer 


THE   INNER    PLANETS  87 

the  sun  than  would  otherwise  have  been  the  case.  He 
thus  selected  the  best  possible  opportunity.  To  guard 
against  any  subsequent  bias  on  the  part  of  the  examiner 
of  the  plates,  after  the  spectroscope  had  taken  a  plate  it 
was  then  reversed,  and  the  process  repeated  on  another 
one,  the  iron  being  sparked  as  before.  What  had  been 
the  right  side  of  Venus  with  regard  to  the  red  end 
of  the  spectrum  thus  became  the  left  one,  and  vice 
versa.  In  this  manner,  when  the  plates  came  to  be 
measured  for  tilt,  the  measurer  would  have  no  indi- 
cation from  the  spectrum  itself  which  way  the  lines 
might  be  expected  to  tilt;  he  could,  therefore,  not 
be  influenced  either  consciously  or  unconsciously  in 
his  decision. 

Eight  plates  with  their  comparison  ferric  spectra 
were  thus  secured;  four  with  the  spectroscope  direct, 
four  with  it  reversed.  They  were  then  shuffled,  their 
numbers  hidden,  and  given  to  Dr.  Slipher  to  measure. 
The  spectral  lines  told  their  own  story,  and  without 
prompting.  All  the  plates  agreed  within  the  margin 


SPECTROGRAM  OF  VENUS,  SHOWING  ITS  LONG  DAY  —  V.  M.  SLIPHER, 
LOWELL  OBSERVATORY,  1903. 

of  error  accordant  with  their  possible  precision,  a  pre- 
cision some  thirty  times  that  of  Belopolski's  experiment 
on  the  same  lines,  —  a  result  not  derogatory  of  that 


88         THE   EVOLUTION   OF   WORLDS 

investigator,  but  merely  illustrative  of  superior  equip- 
ment. They  showed  conclusively  that  a  rotation  of 
anything  like  twenty-four  hours  was  out  of  the  ques- 
tion. They  yielded,  indeed,  testimony  to  a  negative 
rotation  of  three  months,  which,  interpreted,  means 
that  so  slow  a  spin  as  this  was  beyond  their  power 
to  precise. 

For  Dr.  Slipher  was  at  no  less  care  to  determine  just 
what  precision  was  possible  in  the  case,  although  a 
speed  corresponding  to  a  spin  of  twenty-four  hours  on  a 
globe  the  size  of  Venus  is  well  known  to  be  spectroscop- 
ically  measurable.  It  would  mean  a  motion  toward  us 
of  one  thousand  miles  an  hour,  or  about  a  third  of  a  mile 
a  second.  The  tilt  occasioned  by  this  speed  is  well  within 
the  spectroscope's  ability  to  disclose.  Not  content  with 
this,  however,  by  two  special  investigations,  he  proved  the 
spectroscope's  actual  limits  of  performance  to  be  far 
within  the  quantity  concerned.  One  of  them  was  the 
determination  by  the  same  means  and  in  like  manner  of 
the  rotation  time  of  Mars,  the  length  of  that  planet's 
day,  which  in  other  ways  we  know  to  the  hundredth  of 
a  second,  and  which  is  24*  37m  23'.  66.  Now  Mars  offers 
a  test  nearly  twice  as  difficult  as  Venus,  even  supposing 
the  apparent  disks  of  the  two  the  same,  because  his  diam- 
eter being  less  in  the  proportion  roughly  of  one-half, 
the  actual  speed  of  a  particle  at  his  edge  is  less  for  the 
same  time  of  rotation  in  the  like  proportion,  and  it  is  only 


THE   INNER    PLANETS  89 

with  the  speed  in  miles,  not  in  angular  amount,  that  the 
spectroscope  is  concerned.  Nevertheless,  when  a  like 
number  of  plates  were  tried  on  him,  they  indicated  on 
measurement  a  rotation  time  within  an  hour  of  the  true. 
This  corresponds  to  half  an  hour  on  Venus.  We  see, 
therefore,  that  had  Venus'  day  been  anywhere  in  the 
neighborhood  of  twenty-four  hours,  Dr.  Slipher's  in- 
vestigation would  have  disclosed  it  to  within  thirty-one 
minutes. 

This  result  was  further  borne  out  by  a  similar  test 
made  by  him  of  Jupiter.     Inasmuch  as  the  diameter  of 


SPECTROGRAM  OF  JUPITER,  GIVING  THE  LENGTH  OF  ITS  DAY  BY  THE  TILT  OF 

ITS  SPECTRAL  LINES  —  V.   M.   SLIPHER,    LOWELL   OBSERVATORY. 

Jupiter  is  twelve  times  that  of  Venus,  while  the  rotation 
time  is  Qh  5om.4  at  the  equator,  the  precision  attained 
on  Venus  should  here  have  been  about  a  minute.  And 
this  is  what  resulted.  Slipher  found  the  rotation  time 
spectrographically  Qh  50™,  or  in  accordance  with  the 
known  facts,  while  previous  determinations  with  the 
spectroscope  had  somehow  fallen  short  of  it. 

The  care  at  Flagstaff  with  which  the  possibility  of 
error  was  sought  to  be  excluded  in  this  investigation  of 
the  length  of  Venus'  day  and  the  concordant  precision 


9o         THE   EVOLUTION   OF   WORLDS 

in  the  results  are  worthy  of  notice.  For  it  is  by  thus 
being  particular  and  systematic  that  the  accuracy  of  the 
determinations  made  there,  in  other  lines  besides  this, 
has  been  secured. 

In  size,  Venus  of  all  the  planets  most  nearly  ap- 
proaches the  Earth.  She  is  7630  miles  in  diameter  to 
the  Earth's  7918.  Her  density,  too,  is  but  just  inferior 
to  ours.  And  she  stands  next  us  in  place,  closest  in 
condition  and  constitution  in  the  primal  nebula.  Yet 
in  her  present  state  she  could  hardly  be  more  diverse. 
This  shows  us  how  dangerous  it  is  to  dogmatize  upon 
what  can  or  cannot  be,  and  how  enlightening  beyond 
expectation  often  is  prolonged  and  systematic  study  of 
the  facts. 

The  next  planet  outward  is  our  own  abode.  It  is  one 
of  which  most  of  us  think  we  know  considerable  from  ex- 
perience and  yet  about  which  we  often  reason  cosmically 
so  ill.  If  we  knew  more,  we  should  not  deem  ourselves 
nearly  so  unique.  For  we  really  differ  from  other  mem- 
bers of  our  system  not  more  than  they  do  from  one  an- 
other. Much  that  appears  to  us  fundamental  is  not  so 
in  fact.  Thus  many  things  which  seem  matters  of 
course  are  merely  accidents  of  size  and  position.  Our 
very  day  and  night  upon  which  turn  the  habits  of  all 
animals  and,  even  in  a  measure,  those  of  plants,  are,  as  we 
have  seen,  not  the  possession  of  our  nearest  of  cosmic  kin. 
Our  seasons  which  both  vegetally  and  vitally  mean  so 


THE   INNER   PLANETS  91 

much  are  absent  next  door.  And  so  the  list  of  our 
globe's  peculiar  attributes  might  be  run  through  to  the 
finding  of  diversity  to  our  familiar  ways  at  every  turn. 
But,  as  we  shall  see  later,  these  differences  from  one 
planet  to  the  next  are  not  only  not  incompatible  with  a 
certain  oneness  of  the  whole,  but  actually  help  to  make 
the  family  relationship  discoverable.  Analogy  alone 
is  a  dangerous  guide,  but  analogy  crossed  with  diver- 
sity is  of  all  clews  the  most  pregnant  of  understanding. 
The  very  fact  that  we  can  tell  them  apart  when  we  see 
them  together,  as  the  Irishman  remarked  of  two  brothers 
he  was  in  the  habit  of  confusing,  points  to  their  brotherly 
relation. 

Proceeding  still  further,  we  come  to  Mars  at  a  mean 
distance  of  one  hundred  and  forty-one  million  miles. 
Smaller  than  ourselves,  his  diameter  is  but  a  little  over 
half  the  Earth's,  or  forty-two  hundred  miles,  his  mass 
one-ninth  of  ours,  and  his  density  about  seven-tenths 
as  much.  Here,  again,  but  in  a  different  way,  we 
find  a  planet  unlike  ourselves,  and  we  know  more  about 
him  than  of  any  body  outside  the  Earth  and  Moon. 
So  much  about  him  has  been  set  forth  elsewhere  that  it 
is  enough  to  mention  here  that  no  oceans  diversify  his 
surface,  no  mountains  relieve  it,  and  but  a  thin  air 
wraps  it  about,  —  an  air  containing  water-vapor,  but 
so  clear  that  the  surface  itself  is  almost  never  veiled 
from  view. 


92         THE   EVOLUTION   OF  WORLDS 

About  the  satellites  Mars  possesses,  Deimos  and  Pho- 
bos,  we  may  perhaps  say  a  word,  as  recent  knowledge 
concerning  them  exemplifies  the  care  now  taken  to  such 
ascertainment  and  the  importance  of  considering  factors 
often  overlooked.  Soon  after  they  were  discovered  in 
1877,  they  were  measured  photometrically,  with  the 
result  of  giving  a  diameter  of  six  miles  to  Deimos  and 
one  of  seven  miles  to  Phobos,  and  these  values  un- 
challenged entered  the  text-books.  When  the  satellites 
came  to  be  critically  considered  at  Flagstaff,  it  was 
found  that  these  determinations  were  markedly  in  error, 
Phobos  being  very  much  the  larger  of  the  two,  the  actual 
values  reaching  nearer  ten  miles  for  Deimos  and  thirty- 
six  for  Phobos. 

In  getting  the  Flagstaff  values,  the  size  to  the  eye 
of  the  satellite  was  corrected  for  the  background  upon 
which  it  shone;  for  the  background  is  all-important  to 
the  brilliancy  of  a  star.  In  the  case  of  a  small  star  near 
a  planet,  the  swamping  glare  of  the  planet  is  something 
like  the  inverse  cube  of  its  distance  away.  Furthermore, 
the  Flagstaff  observations  indicated  how  the  previous 
error  had  crept  in.  For  before  correction  for  the  dif- 
fering brilliancies  of  the  field  of  view,  the  apparent 
size  of  the  satellites  judged  by  conspicuousness  was 
about  six  to  seven.  The  photometric  values  must  have 
been  taken  just  as  they  came  out,  no  correction  appar- 
ently having  been  made  for  the  background.  Now  the 


THE   INNER   PLANETS  93 

background  is  a  fundamental  factor  in  all  photometric 
determinations,  a  factor  somewhat  too  important  in  this 
case  to  neglect,  since  it  affected  the  result  2500  per 
cent. 


CHAPTER   IV 

THE    OUTER    PLANETS 

BEYOND  Mars  lies  the  domain  of  the  asteroids,  a 
domain  vast  in  extent,  that,  untenanted  by  any 
large  planet,  stretches  out  to  Jupiter.  Occupied  solely 
by  a  host  of  little  bodies  agreeing  only  in  lack  of  size, 
even  this  space  seems  too  small  to  contain  them,  for 
recent  research  has  shown  some  transgressing  its 
bounds.  One,  Eros,  discovered  by  De  Witt,  more 
than  trenches  on  Mars'  territory,  having  an  orbit 
smaller  than  that  of  the  god  of  war,  and  may  be  con- 
sidered perhaps  the  forerunner  of  more  yet  to  be  found 
between  Mars  and  the  Earth.  On  the  other  side, 
three  recently  detected  by  Max  Wolf  at  Heidelberg 
have  periods  equal  to  that  of  Jupiter,  and  in  their 
motions  appear  to  exemplify  an  interesting  case  of  celes- 
tial mechanics  pointed  out  theoretically  by  Lagrange 
long  before  its  corroboration  in  fact  was  so  much  as 
dreamt.  Achilles,  Patroclus,  and  Hector,  as  the  triad 
are  called,  so  move  as  always  to  keep  their  angular 
distance  from  Jupiter  unaltered  in  their  similar  circuits 
of  the  Sun. 

Before  considering  these  bodies  individually,  we  may 

94 


THE   OUTER   PLANETS 


95 


well  look  upon  them  en  bloc,  inasmuch  as  one  attribute 
of  the  asteroids  concerns  them  generically  rather  than 
specifically,  and  is  of  great  interest  both  from  a  mechani- 


ORBITS  OF  THE  OUTER  PLANETS. 

cal  and  an  historical  point  of  view.  For,  in  fact,  it  is 
what  led  to  their  discovery.  Titius  of  Wittenburg, 
about  the  middle  of  the  eighteenth  century,  noticed  a 
curious  relation  between  the  distances  from  the  Sun  of 
the  then  known  planets.  It  consisted  in  a  sort  of  regu- 
lar progression,  but  with  one  significant  gap.  Bode  was 
so  struck  by  the  gap  that  he  peopled  it  with  a  supposed 


96         THE   EVOLUTION  OF  WORLDS 

planet,  and  so  brought  the  relation  into  general  regard 
in  1772.  In  consequence,  it  usually  bears  his  name. 
It  is  this :  if  we  take  the  geometrical  series,  3,  6,  12,  24, 
48,  96  and  add  4  to  each  term,  we  shall  represent  to  a 
fair  degree  of  precision  the  distances  of  the  several 
planets,  beginning  with  Mercury  at  4  and  ending  with 
Saturn  at  100,  which  was  the  outermost  planet  then 
known.  All  the  terms  were  represented  except  24  +  4, 
or  28  —  a  gap  lying  between  Mars  and  Jupiter.  When 
Uranus  was  discovered  by  Sir  William  Herschel  in  1781 
and  was  found  to  be  travelling  at  what  corresponded 
to  the  next  outer  term  192  +  4,  or  196,  the  opinion  be- 
came quite  general  that  the  series  represented  a  real 
law  and  that  28  must  be  occupied  by  a  planet.  Von 
Zach  actually  calculated  what  he  called  its  analogical 
elements,  and  finally  got  up  in  1800  a  company  to  look 
for  it  which  he  jocularly  described  as  his  celestial 
police.  Considering  that  Bode's  law  is  not  a  law  at  all, 
but  a  curious  coincidence,  as  Gauss  early  showed  in  its 
lack  of  precision  and  in  its  failure  to  mark  the  place  of 
Mercury  with  any  approach  to  accuracy,  and  as  the 
discovery  of  Neptune  amply  bore  out,  it  was  perhaps 
just  in  fate  that  the  honor  of  filling  the  gap  did  not  fall 
to  any  of  the  "  celestial  police,"  but  to  an  Italian 
astronomer,  Piazzi,  at  the  time  engaged  on  a  new  star 
chart.  An  illness  of  Piazzi  caused  it  to  be  lost  almost 
as  soon  as  found.  In  this  plight  an  appeal  was  made 


THE   OUTER   PLANETS  97 

to  the  remarkable  Gauss,  just  starting  on  his  career. 
Gauss  undertook  the  problem  and  devised  formulae 
by  which  its  place  was  predicted  and  the  planet  itself 
recovered.  It  proved  to  fit  admirably  the  gap.  But 
it  had  hardly  been  recovered  before  another  planet 
turned  up  equally  filling  the  conditions.  Ceres,  the 
first,  lay  at  26.67  astronomical  units  from  the  Sun; 
Pallas,  the  second,  at  27.72.  Two  claimants  were  one 
too  many.  But  the  inventive  genius  of  Olbers  came 
to  the  rescue.  By  a  bold  hypothesis  he  suggested 
that  since  two  had  appeared  where  only  one  was 
wanted,  both  must  originally  have  formed  parts  of 
a  single  exploded  planet.  He  predicted  that  others 
would  be  detected  by  watching  the  place  where  the 
explosion  had  occurred,  to  wit:  where  the  orbits  of 
Ceres  and  Pallas  nearly  intersected  in  the  signs  of  the 
Virgin  and  the  Whale. 

For  in  the  case  of  an  explosion  the  various  parts, 
unless  perturbed,  must  all  return  in  time  to  the  scene 
of  the  catastrophe.  By  following  his  precept,  two  more 
were  in  fact  detected  in  the  next  two  years,  Juno  and 
Vesta.  His  hypothesis  seemed  to  be  confirmed. 
No  new  planets  were  discovered,  and  the  old  fulfilled 
fairly  what  was  required  of  them.  Lagrange  on  cal- 
culation gave  it  his  mathematical  assent. 

Nevertheless,  it  was  incorrect,  as  events  eventually 
showed,  though  for  forty  years  it  slept  in  peace,  no 


98         THE   EVOLUTION   OF   WORLDS 

new  asteroids  being  found.  We  now  know  that  this 
was  because  the  rest  were  all  much  smaller,  and  for 
such  nobody  looked.  It  was  not  till  1845  tnat  Hencke, 
an  ex-postmaster  of  Driessen  in  Prussia,  after  fifteen 
years  of  search  detected  another,  Astraea,  of  the  nth 
magnitude.  After  this  discoveries  of  them  came  on 
apace,  until  now  more  than  six  hundred  are  known, 
and  their  real  number  seems  to  be  legion.  But  those 
discovered  are  smaller  each  year  on  the  average, 
showing  that  the  larger  have  already  been  found. 
Their  orbits  are  such  that  they  cannot  possibly  ever 
have  all  formed  part  of  a  pristine  whole.  The  idea, 
not  the  body,  was  exploded.  For  they  are  now  recog- 
nized as  having  always  been  much  as  they  are  to-day. 
They  prove  to  be  thickest  at  nearly  the  point  where 
Bode's  law  required,  the  spot  where  Ceres  and  Pallas 
were  found.  The  mean  of  their  distances  is  less,  being 
2.65  instead  of  2.8  astronomical  units,  probably  simply 
because  the  nearer  ones  are  easier  discovered.  The 
fact  that  they  are  clustered  most  thickly  just  inside 
2.8  astronomical  units  implies  that  there  of  all  points 
within  the  space  between  Mars  and  Jupiter  a  planet 
would  have  formed  if  it  could.  A  definite  reason 
exists  for  its  failure  to  do  so  —  Jupiter's  disturbing 
presence.  Throughout  this  whole  region  Jupiter's 
influence  is  great;  so  great  that  his  scattering  effect 
upon  the  particles  exceeds  their  own  tendency  to  come 


THE   OUTER   PLANETS  99 

together.  We  see  this  in  the  arrangement  of  the 
orbits.  If  we  plot  the  orbits  of  the  asteroids,  we  shall 
be  struck  by  the  emergence  of  certain  blanks  in  the 
ribbon  representing  sections  of  their  path.  It  is  the 
woof  of  a  plaid  of  Jupiter's  weaving.  The  gaps  are 
where  asteroids  revolving  about  the  Sun  would  have 
periods  commensurate  with  his,  J- ,  ^,  J-,  ^,  and  the  like. 
Such  bodies  would  return  after  a  few  revolutions, 
five  of  theirs,  for  instance,  to  Jupiter's  two,  into  the 
same  configurations  with  him  at  the  same  points  of 
their  orbits.  Thus  the  same  perturbation  would  be 
repeated  over  and  over  again  until  the  asteroid's  path 
was  so  changed  that  commensurability  ceased  to 
exist.  And  it  would  be  long  before  perturbation 
brought  it  back  again.  Thus  the  orbits  are  constantly 
swinging  out  and  in,  all  of  them  within  certain  limits, 
but  those  are  most  disturbed  which  synchronize  with 
his.  In  this  manner  he  has  fashioned  their  arrange- 
ment and  even  prevented  any  large  planet  from  form- 
ing in  the  gap. 

Such  restrictive  action  is  not  only  at  work  to-day 
in  the  distribution  of  the  asteroids  and  in  the  partitions 
of  Saturn's  ring,  but  it  must  have  operated  still  more 
in  the  past  while  the  system  was  forming.  To  Pro- 
fessor Milham  of  Williamstown  is  due  the  brilliant 
suggestion  that  this  was  the  force  that  fashioned  the 
planetary  orbits.  For  a  planet  once  given  off  from  a 


ioo       THE   EVOLUTION   OF  WORLDS 

central  mass  would  exercise  a  prohibitive  action  upon 
any  planet  trying  to  form  within.  In  certain  places 
it  would  not  allow  it  to  collect  at  all.  The  evolution 
of  the  solar  family  would  resemble  that  of  some  human 
ones  in  which  each  child  brings  up  the  next  in  turn. 
So  that  the  planetary  system  made  itself,  as  regards 
position,  a  steadily  accumulative  set  of  prohibitions 
combining  to  leave  only  certain  places  tenantable. 

In  this  manner  we  may  perhaps  be  brought  back 
to  Bode's  law  as  representing  within  a  certain  degree 
of  approximation  a  true  mechanical  result,  although 
no  such  exact  relation  as  the  law  demands  exists. 
That  a  relation  seemingly  close  to  it  is  necessitated  by 
the  several  successive  inhibitions  of  each  planet  upon 
the  next  to  form,  is  quite  possible. 

One  other  general  trait  about  their  orbits  is  worth 
animadversion.  In  spite  of  being  eccentric  and  in- 
clined, they  are  all  traversed  in  the  same  sense.  Every 
one  of  the  asteroids  travels  direct  like  the  larger  planets. 
In  this  they  differ  from  cometary  paths,  which  are  as 
often  retrograde  as  direct.  Thus  in  more  ways  than 
one  they  hold  a  mid-course  in  regularity  between  the 
steady,  even  character  of  the  planets  proper  and  what 
was  for  long  deemed  the  erratic  behavior  of  the 
cometary  class  of  cosmic  bodies.  Very  telling  this 
fact  will  be  found  with  regard  to  the  genesis  of  the 
solar  family,  as  we  shall  see  later. 


THE   OUTER   PLANETS  101 

With  regard  now  to  their  more  individual  characteris- 
tics, the  asteroids  may  be  said  to  agree  in  one  point  — 
their  diversity,  not  only  to  all  the  larger  members  of  the 
solar  family,  but  to  one  another.  For  they  travel  in  or- 
bits ranging  in  ellipticity  all  the  way  from  such  as  nearly 
approach  circles  to  ellipses  of  cometary  eccentricity. 
They  voyage,  too,  without  regard  to  the  dynamical  plane 
of  the  system,  or,  what  is  close  to  it,  the  ecliptic;  depart- 
ing from  the  general  level  often  30 6  and,  in  one  instance, 
that  of  the  little  planet  dubbed  W.  D.,  by  as  much  as 
48°.  This  eccentricity  and  inclination  put  them  in  a 
class  by  themselves.  It  is  associated  and  unquestionably 
connected  mechanically  with  another  trait  which  like- 
wise distinguishes  them  from  the  planets  more  particu- 
larly called  —  their  diminutive  size.  Only  four — Vesta, 
Ceres,  Pallas,  and  Juno  —  out  of  the  six  hundred  odd 
now  known  exceed  a  hundred  miles  in  diameter,  and  the 
greater  number  are  hardly  over  ten  or  twenty  miles 
across.  Very  tiny  worlds  indeed  they  would  seem,  could 
we  get  near  enough  to  them  to  discern  their  forms  and 
features.  Curiously  enough,  reasoning  on  certain  light 
changes  they  exhibit  has  enabled  us  to  divine  something 
of  their  shapes,  and  even  character.  Thus  it  was  soon 
perceived  that  Eros  fluctuated  in  the  light  he  sent 
us,  being  at  times  much  brighter  than  at  others.  In 
February  and  March,  1901,  the  changes  were  such  that 
their  maximum  exceeded  three  times  their  minimum  two 


102        THE   EVOLUTION   OF   WORLDS 

hours  and  a  half  later.  Then  in  May  the  variation  van- 
ished. More  than  one  explanation  has  been  put  for- 
ward, but  the  best  so  far,  because  the  most  simple,  is 
that  the  body  is  not  a  sphere  but  a  jagged  mass,  a  moun- 
tain alone  in  space,  and  that  as  it  turns  upon  its  axis 
first  one  corner  and  then  another  is  presented  to  our 
view  or  throws  a  shade  upon  its  neighbor.  When  the 
pole  directly  faces  us,  no  great  change  occurs,  especially 
if  it  also  nearly  faces  the  Sun.  Yet  even  this  fails  to 
explain  all  its  vagaries. 

Eros  is  not  alone  in  thus  exhibitingvariation.  Sirona, 
Hertha,  and  Tercidina  have  also  shown  periodic  vari- 
ability, and  it  is  suspected  in  others.  Indeed,  it  would 
be  surprising  did  they  not  show  change.  For  they  are 
too  small  to  have  drawn  their  contents  into  symmetry, 
and  so  remain  as  they  were  when  launched  in  space. 
Mammoth  meteorites  they  undoubtedly  are. 

With  the  asteroids  we  leave  the  inner  half  of  the  Sun's 
retinue  and  pass  to  the  outer.  Indeed,  the  asteroids 
not  only  mark  in  place  the  transition  bound  between 
the  two,  but  stamp  it  such  mechanically.  In  their 
own  persons  they  witness  that  no  large  body  was  here 
allowed  to  form.  The  culmination  of  coalition  was 
reached  in  Jupiter,  and  that  very  acme  of  accretion 
prevented  through  a  long  distance  any  other. 

In  bulk,  the  major  planets  compared  with  the  inner  or 
terrestrial  ones  form  a  class  apart;  and  among  the  major 


THE   OUTER    PLANETS  103 

Jupiter  is  by  all  odds  first.  His  mass  is  318  times  the 
Earth's  and  his  volume  nearly  1400  times  hers.  From 
this  it  appears  that  his  density  is  very  much  less.  In- 
deed, his  substance  is  only  fractionally  denser  than  water. 
This  and  its  tremendous  spin,  carrying  a  point  at  its 
equator  two  hundred  and  eighty  thousand  miles  round 
in  less  than  ten  hours,  flatten  it  to  a  very  marked 
oval  with  an  ellipticity  of  1-5-75-- 
Not  the  least  beautiful  of  the 
revelations  of  astronomy  are 
the  geometrical  shapes  of  the 
heavenly  bodies,  proceeding 
from  nearly  perfect  spheres 
like  the  Sun  or  Moon  to 

marked    spheroids    like    Jupi-        DRAWING  OF  JUPITER  BY  DR. 

LOWELL.    APRIL  12,  1907. 

ter  or  Saturn.     So  enormous 

are  the  masses  and  the  forces  concerned  that  the  forms 
assumed  under  them  are  mechanically  regular.  They 
are  the  visible  expression  of  gravitation,  and  so  delight 
the  brain  while  they  satisfy  the  eye. 

It  is  to  appreciation  of  the  detail  visible  on  Jupiter's 
disk  that  modern  advance  in  the  study  of  the  planet 
is  indebted.  Examination  has  shown  its  features 
to  be  of  great  interest.  To  Mr.  Stanley  Williams  of 
Brighton,  England,  much  of  our  knowledge  is  due, 
and  Mr.  Scriven  Bolton  has  also  made  some  interest- 
ing contributions.  The  big  print  of  the  subject,  read 


io4        THE   EVOLUTION   OF   WORLDS 

long  ago,  is  that  the  planet's  disk  is  noticeably  banded 
by  dark  belts.  Two  characteristics  of  these  belts 
are  important.  One  is  that  they  exhibit  a  regular 
secular  progression  with  the  lapse  of  years,  the 
south  tropical  belt  being  broader  and  more  salient  for 
many  years  in  succession,  and  then  gradually  fading 
out  while  the  northern  one  increases  in  prominence. 
It  has  been  suspected  that  the  rhythm  of  their  change 
is  connected  with  that  of  sun  spots.  The  second  is 
that  the  belts  do  not  preserve  in  their  several  features 
the  same  relation  in  longitude  toward  one  another. 
They  all  rotate,  but  at  different  speeds.  There  could 
be  no  better  proof  that  Jupiter  is  no  solid,  but  a  seething 
mass  of  heavy  vapors  boiling  like  a  caldron.  Tem- 
pered by  distance  we  can  form  but  a  faint  idea  of  the 
turmoil  there  going  on.  Further  indication  of  it  is 
furnished  by  its  glow.  For  all  the  dark  belts  are  a 
beautiful  cherry  red,  a  tint  extending  even  to  the 
darkish  hoods  over  the  planet's  caps.  This  hue 
comes  out  well  in  good  seeing,  and  best,  as  with  all 
planetary  markings,  in  twilight,  not  at  night,  because 
the  excessive  brightness  of  the  disk  is  then  taken  off, 
preventing  the  colors  from  being  swamped. 

This  brings  us  to  the  planet's  albedo,  which  Miiller 
at  Potsdam  has  found  to  be  75  per  cent.  Now  the 
interest  attaching  to  this  determination  is  twofold, 
that  it  bespeaks  cloud  and  that  it  seems  to  imply 


THE   OUTER    PLANETS 


105 


JUPITER  AND  ITS  WISPS.  —  A  DRAW- 
ING BY  DR.  LOWELL,  APRIL  n,  1907. 


something  else.  The  albedo  of  cloud  is  72  per  cent 
of  absolute  whiteness.  What  looks  like  cloud,  then, 
is  such,  on  that  distant 
disk.  But  Jupiter  sur- 
passes cloud  in  lustre,  since 
his  albedo  exceeds  72  per 
cent.  Yet  a  large  part  of 
his  surface  is  strikingly 
darker  than  that.  The  in- 
ference from  this  is  that  he 
shines  by  intrinsic  light,  in 
part  at  least.  The  fact 
may  not  be  stated  dogmatically,  as  there  is  no  astro- 
nomic determination  so  uncertain  as  this  one  of  deter- 
mining albedoes,  and  therefore  Herr  Miiller's  results 

must  be  accepted  with  every 
reserve,  but  they  suggest 
that  Jupiter  is  still  a  semi- 
sun,  to  be  recognized  as  such 
by  light  as  well  as  heat, 
though  his  self-luminosity, 
if  it  exist  at  all,  can  hardly 
exceed  a  dull  red  glow. 

A  modern  detection  on 
Jupiter's  disk  has  been  that 
of  wisps  or  lacings  across  the  bright  equatorial  belt, 
a  detail  of  importance  due  to  Mr.  Scriven  Bolton. 


JUPITER  AND  ITS  WISPS.  —  A  DRAW- 
ING BY  DR.  LOWELL,  APRIL  n,  1907. 


io6        THE   EVOLUTION   OF  WORLDS 

Requested  to  look  for  them,  the  observatory  at  Flag- 
staff was  not  long  in  corroborating  this  interesting 
phenomenon.  The  peculiarity  about  them  pointed  out 
by  Mr.  Bolton  is  that  they  traverse  the  belt  at  an  angle 
of  about  45°  to  the  vertical,  proceeding  from  caret- 
shaped  dark  spots  projecting  into  the  bright  belt 
from  the  dark  ones  on  either  side.  They  exist  all 
round  the  equator  and  are  found  indifferently  dex- 
trous or  sinister — sometimes  vertical.  For  there  are 
others  that  go  straight  across.  Nor  are  they  confined 
to  the  bright  equatorial  belt,  but  are  to  be  seen  travers- 
ing all  of  the  bright  belts  both  north  or  south  up  to 
the  polar  hoods.  From  its  sombreness  it  seems  that 
we  are  here  regarding  a  phenomenon  in  the  negative; 
remarking  it  by  what  it  has  left  behind,  not  by  what  it 
has  accomplished.  For  the  wisps  are  not  wisps  of 
cloud,  since  they  are  dark,  not  light,  but  gaps  strung 
out  in  the  clouds  themselves. 

Recently  photographs  of  Jupiter  have  been  secured 
at  Flagstaff,  by  the  new  methods  there  of  planetary 
photography,  showing  a  surprising  amount  of  detail. 
The  wisps  come  out  with  certainty,  and  the  white 
spots,  which  are  such  a  curious  feature  of  the  disk,  have 
also  left  their  impress  on  the  plate.  Not  the  least  of 
the  services  thus  rendered  by  the  camera  is  the  accurate 
positioning  of  the  belts  made  possible  by  it.  Micro- 
metric  measures  are  all  very  well  when  nothing  better 


THE   OUTER   PLANETS 


107 


is  attainable,  but  any  one  who  has  made  such  upon  a 
planet's  disk  swinging  like  a  lantern  in  the  field  of 
view  under  a  variety  of  causes  instrumental  and 
optical,  knows  how  encumbered  they  inevitably  are 
with  error.  To  have  the  disk  caught 
and  fixed  on  a  plate  where  it  may  be 
measured  at  leisure  and  as  often  as 
one  likes,  is  a  distinct  advance  toward 
fundamental  accuracy.  Measures  thus 
effected  upon  the  Jupiter  images  of 
1909  proved  the  bright  equatorial  belt 
to  lie  exactly  upon  the  planet's  equator 
when  allowance  was  made  for  the  tilt 
of  the  planet's  axis  toward  the  Earth. 
This  showed  that  the  aspect  of  the 
planet  toward  the  Sun  had  no  effect 
upon  the  position  of  the  belt.  Jupiter's 
cloud  formation,  therefore,  is  not  depend- 
ent, as  all  ours  are,  upon  the  solar  heat. 

A  like  indifference  to  solar  action  is  exhibited  in  the 
utter  obliviousness  of  the  belts  to  day  or  night.  To 
them  darkness  and  light  are  nugatory  alike.  They  re- 
appear round  the  sunrise  edge  of  the  disk  just  as  they 
left  it  when  they  sank  from  sight  round  the  sunset  one, 
and  they  march  across  its  sunlit  face  without  so  much 
as  a  flicker  on  their  features. 

Yet  this  seeming  immobility  from  moment  to  moment 


N. 

PHOTOGRAPH  OF 

JUPITER,  1909. 

P.  L. 


io8        THE   EVOLUTION   OF  WORLDS 

takes  place  in  what  is  really  a  seething  furnace,  the 
fiery  glow  of  which  we  catch  below  the  vast  ebullition  of 
cloud  in  the  cherry  hue  of  its  darker  portions.  Distance 
has  merged  the  turmoil  into  the  semblance  of  quiescence 
and  left  only  its  larger  secular  changes  to  show.  Even 
so  the  Colorado  River  from  the  brink  of  the  Grand  Canon 
is  seen  apparently  at  rest,  the  billows  of  its  rapids  so 
stereotyped  to  stability  one  takes  the  rippled  sand  bank 
for  the  river  and  the  billows  of  the  river  for  the  ripple 
marks  of  its  banks. 

At  twice  the  distance  of  Jupiter  we  cross  the  orbit 
of  Saturn.  Here  the  ringed  planet,  with  an  annual 
sweep  of  twenty-nine  and  a  half  of  our  years,  pursues 
his  majestic  circuit  of  the  Sun.  Diademed  with  three 
or  more  circlets  of  light  and  diamonded  by  ten  satel- 
lites, he  rivals  in  his  cortege  that  of  his  own  lord.  In 
some  ways  his  surpasses  the  Sun's.  For  certainly  his 
retinue  is  the  more  spectacular  of  the  two;  the  more  so 
that  it  is  much  of  it  fairly  comprised  within  a  single 
glance.  Very  impressive  Saturn  is  as,  attended  thus, 
he  sails  into  the  field  of  view. 

In  our  survey  we  may  best  begin  with  his  globe.  If 
Jupiter's  compression  is  striking,  Saturn's  is  positively 
startling  when  well  displayed.  This  happens  but  at  rare 
intervals.  As  the  plane  of  his  equator  is  almost  exactly 
that  of  the  rings,  the  flattening  is  conspicuous  only  on 
those  occasions  when  the  rings  disappear  because  their 


THE   OUTER   PLANETS  109 

plane  passes  through  the  line  of  sight.  Seen  at  such 
times  the  effect  of  the  discrowned  orb  is  so  strange  as  to 
suggest  delusion.  This  occurred  two  years  ago  in  1907, 
and  when  the  planet  was  picked  up  by  its  position  and 
entered  the  field  unheralded  by  its  distinctive  append- 
age, it  was  almost  impossible  to  believe  there  had  not 
been  some  mistake  and  a  caricatured  Jupiter  had 
taken  its  place.  For  the  flattening  outdoes  that  of 
Jupiter  as  3  to  2,  being  ^  of  the  equatorial  diameter. 
Such  a  bulging  almost  suggests  disruption  and  is  due 
to  the  extreme  lightness  of  the  planet's  substance, 
which  is  actually  only  0.72  of  that  of  water.  Like 
Jupiter,  the  disk  exhibits  belts,  though  very  much 
fainter,  and,  like  his,  these  are  of  a  cherry  red.  As 
the  planet's  albedo  is  even  greater,  0.78  of  absolute 
whiteness,  as  deduced  from  H.  Struve's  measures  of 
the  diameter,  the  same  suspicion  of  shining,  at  least  in 
part,  from  inherent  light,  applies  equally  to  him.  But 
it  is  practically  certain  that  in  neither  case  does  this 
light  equal  that  of  the  planet's  clouds,  or  add  anything 
to  them.  Both  planets  are  red-hot,  not  white-hot. 
The  determination  of  the  albedo  depends  upon  that  of 
the  diameter,  and  an  increase  in  the  latter  would  lower 
the  albedo  to  that  of  cloud. 

His  most  unique  possession  are  his  rings.  Broad,  yet 
tenuous,  they  weigh  next  to  nothing,  being,  as  Struve 
has  dubbed  them,  "  Immaterial  light."  Nevertheless, 


i io        THE   EVOLUTION   OF  WORLDS 

it  is  not  their  lightness  but  their  make-up  that  prevents 
from  lying  uneasy  the  head  that  wears  this  crown. 

The  mechanical  marvel  was  not  appreciated  by  early 
astronomers,  who  took  it  for  granted  that  they  were 
what  they  seemed,  solid,  flat  rings,  all  of  a  piece.  Even 
Laplace  considered  it  sufficient  to  divide  them  up  con- 
centrically to  insure  stability.  To  Edouard  Roche  of 
Montpellier,  as  retiringly  modest  as  he  was  penetrat- 
ingly profound,  is  due  the  mathematical  detection  that 
to  subsist  they  must  be  composed  of  discrete  particles, 
-brickbats,  Clerk  Maxwell  called  them,  when,  later, 
unaware  of  Roche's  work,  he  proved  independently 
the  same  thing  in  his  essay  on  Saturn's  rings.  Peirce, 
too,  in  ignorance  of  Roche,  had  half  taken  the  same 
step  a  little  before,  showing  that  they  must  at  least  be 
fluid.  Then  in  1895  Keeler  ingeniously  photographed 
the  spectrum  of  both  ball  and  rings  to  the  revealing  of 
velocities  in  the  line  of  sight  of  the  different  portions  of 
the  spectrum  exactly  agreeing  with  the  values  me- 
chanics demanded. 

The  rings  have  usually  been  considered  to  be  flat. 
At  the  time  of  their  disappearance,  however,  knots 
have  been  seen  upon  them.  It  is  as  if  their  filament 
had  suddenly  been  strung  with  beads.  At  the  last 
occurrence  of  the  sort  in  1907,  these  beads  were  partic- 
ularly well  seen  at  several  observatories,  and  were 
critically  studied  at  Flagstaff.  In  connection  with  a 


THE   OUTER    PLANETS  in 

new  phenomenon  detected  there,  that  of  a  dark  core 
in  the  shadow  the  rings  threw  across  the  planet's  face, 
an  explanation  suggested  itself  to  account  for  both 
them  and  it:  to  wit,  that  the  rings  were  not  really 
flat,  but  tores;  rings,  that  is,  like  an  anchor  ring,  any 
cross-section  of  which  would  be  of  the  nature  of  an 
oval  flattened  on  its  inner  side.  The  cogency  of  the 
explanation  consisted  in  its  solution  not  only  of  the 
appearances  but  of  the  cause  competent  to  bring  those 
appearances  about. 

For  measurement  showed  that  the  knots  were  per- 
manent in  position,  which,  since  the  ring  revolved, 
indicated  that  they  extended  all  round  it  in  spite  of 
their  not  seeming  to  do  so,  and  that  their  distances 
from  Saturn  were  just  what  this  cause  should  pro- 
duce. 

The  action  observed  was  a  corollary  from  the 
important  principle  of  commensurability  of  orbital 
period.  As  we  saw  in  the  case  of  the  asteroids,  if  two 
bodies  be  travelling  round  a  third  and  their  respec- 
tive periods  of  revolution  be  commensurate,  they  will 
constantly  meet  one  another  in  such  a  manner  that 
great  perturbation  will  ensue  and  the  bodies  be  thrown 
out  of  commensurability  of  period. 

What  has  happened  to  the  asteroids  has  likewise 
occurred  in  Saturn's  rings.  The  disturber  in  this  case 
has  been,  not  Jupiter,  as  with  them,  but  one  or  other 


ii2        THE   EVOLUTION   OF  WORLDS 

of  Saturn's  own  satellites.  For  when  we  calculate  the 
problem,  we  find  that  Mimas,  Enceladus,  and  Tethys 
have  periods  exactly  commensurate  with  the  divisions 
of  the  rings;  in  other  words,  these  three  inner  satellites, 
whose  action  because  of  proximity  is  the  greatest, 
have  fashioned  the  rings  into  the  three  parts  we  know, 
called  A,  the  outermost;  B,  the  middle  one;  and  C,  the 
crepe  ring,  nearest  to  the  body  of  the  planet.  Mimas 
has  been  the  chief  actor,  though  helped  by  the  two 
others,  while  Enceladus  has  further  subdivided  ring  A 
by  what  is  known  as  Encke's  division. 

Such  has  been  the  chief  action  of  the  satellites  on  the 
rings :  it  has  made  them  into  the  system  we  see.  But 
if  we  consider  the  matter,  we  shall  realize  that  a  second- 
ary result  must  have  ensued  —  when  we  remember  that 
the  particles  composing  the  rings  must  be  very  crowded 
for  the  rings  to  show  as  bright  as  they  do,  and  also 
that,  though  relatively  thin,  the  rings  are  nevertheless 
some  eighty  miles  through. 

Now  it  is  evident  that  any  disturbance  in  so  closely 
packed  a  system  of  small  bodies  as  that  constituting 
Saturn's  rings  must  result  in  collisions  between  the 
bodies  concerned.  Particles  pulled  out  or  in  must 
come  in  contact  with  others  pursuing  their  own  paths, 
and  as  at  each  collision  some  energy  is  lost  by  the 
blow,  a  general  falling  in  toward  the  planet  results. 
At  the  same  time,  as  the  blow  will  not  usually  be  exactly 


THE   OUTER    PLANETS 


in  the  plane  in  which  either  particle  was  previously 

moving,  both  will  be  thrown  more  or  less  out  of  the 

general   plane   of  their   fellows,  and   the   ring  at  that 

point,  even  if  originally  flat,  will  not  remain  so.     For 

the   ring,   though  very 

narrow   relatively,    has 

a  real  thickness,  quite 

sufficient  for  slantwise 

collision,  if  the  bodies 

impinge. 

Now  the  knots  or 
beads  on  the  rings  ap- 
peared exactly  inside 
the  points  where  the 
satellites'  disturbing 
action  is  greatest,  or, 
in  other  words,  in  pre- 
cisely their  theoretic 
place.  We  can  hardly  doubt  that  such,  then,  was 
their  origin.* 

The  result  must  be  gradually  to  force  the  particles  as- 
a  rule  nearer  the  planet,  until  they  fall  upon  its  surface, 
while  a  few  are  forced  out  to  where  they  may  coalesce 
into  a  satellite,  —  a  result  foreseen  long  ago  by  Maxwell. 
It  is  this  process  which  in  the  knots  we  are  actually 
witnessing  take  place,  and  which,  like  the  corona  about 

*  Paper  by  the  writer  in  the  Phil.  Mag.,  April,  1908. 

I 


1 14        THE   EVOLUTION   OF  WORLDS 

the  eclipsed  Sun,  only  comes  out  to  view  when  the 
obliterating  brightness  of  the  main  body  of  the  rings 
is  withdrawn  by  their  edgewise  presentation. 

The  reason  the  out-of-plane  particles  are  most 
numerous  just  inside  the  point  of  disturbance  is  not 
only  that  there  the  action  throwing  them  out  is  most 
violent,  but  that  all  the  time  a  levelling  action  quite 
apart  from  disturbance  is  all  the  time  tending  to 
reduce  them  again  to  one  plane,  as  we  shall  see  further 
on  when  we  come  to  the  mechanical  forces  at  work. 
Thus  the  tore  is  most  pronounced  on  its  outer  edge, 
and  falls  to  a  uniform  level  at  its  inner  boundary. 
The  effect  is  somewhat  as  represented  in  the  adjoining 
cut,  in  which  the  vertical  scale  is  greatly  magnified  :  — 


THE  TORES  OF  SATURN.    Not  drawn  to  scale. 

With  Saturn  ended  the  bounds  of  the  solar  system  as 
known  to  the  civilized  world  until  1781.  On  March  13 
of  that  year  Sir  William  Herschel  in  one  of  his  telescopic 
voyages  through  space  came  upon  a  strange  object  which 
he  at  once  saw  was  not  a  star,  because  of  its  very  percep- 
tible round  disk,  and  which  he  therefore  took  for  a  pecul- 


THE   OUTER   PLANETS  115 

iar  kind  of  comet.  Nearly  a  year  rolled  by  before  Lexell 
showed  by  calculation  of  its  motion  that  it  was  no  comet, 
but  undoubtedly  a  new  planet  beyond  Saturn  travel- 
ling at  almost  twice  that  body's  mean  distance  from 
the  Sun. 

By  reckoning  backward,  it  was  found  to  have  been 
seen  and  mapped  several  times  as  a  star,  —  no  less  than 
twelve  times  by  Lemonnier  alone,  —  and  yet  its  plan- 
etary character  had  slipped  through  his  fingers.  It  can 
even  be  seen  with  the  naked  eye  as  a  star  of  the  6th 
magnitude,  and  its  course  is  said  to  have  been  watched 
by  savage  tribes  in  Polynesia  long  before  Sir  William 
Herschel  discovered  it. 

Its  greenish  blue  disk  indicates  that  it  is  about  thirty- 
two  thousand  miles  in  diameter,  and  its  mass  that  its 
density  is  about  0.22  of  the  Earth's  or,  like  Jupiter's, 
somewhat  greater  than  water.  Of  its  surface  we  prob- 
ably see  nothing.  Indeed,  it  is  very  doubtful  if  it  have 
any  surface  properly  so  called,  being  but  a  ball  of  va- 
pors. Its  flattening,  ^T  according  to  Schiaparelli,  which 
is  probably  the  best  determination,  agrees  with  the  den- 
sity given  above,  indicating  its  substance  to  be  very 
light.  Belts  have  faintly  been  descried  traversing  its 
disk  after  the  analogy  of  Jupiter  and  Saturn.  These 
would  be  much  better  known  than  they  are  but  for  the 
great  tilt  of  the  planet's  axis  to  the  ecliptic,  so  that  during 
a  part  of  its  immense  annual  sweep  its  poles  are  pointed 


u6        THE   EVOLUTION   OF  WORLDS 

nearly  at  the  Earth,  and  its  tropical  features,  the  places 
where  the  belts  lie,  are  wholly  hidden  or  greatly  fore- 
shortened from  our  point  of  view.  As  the  planet's  year 
is  eighty-four  of  our  years  long,  it  is  only  at  intervals 
of  forty  odd  years  that  the  disk  is  well  enough  displayed 
to  bring  the  belts  into  observable  position. 

The  planet  is  attended  by  four  satellites,  —  Ariel,  Um- 
briel,  Titania,  and  Oberon,  —  a  midsummer  night's 
dream  to  a  watcher  of  the  skies.  They  travel  in  a  plane 
inclined  98°  to  the  ecliptic,  so  that  their  motion  is  nearly 
up  and  down  to  that  plane  and  even  a  little  backward. 
Whether  their  plane  is  also  the  equatorial  plane  of  the 
planet,  we  do  not  know  for  certain.  The  observations 
as  yet  are  not  conclusive  one  way  or  the  other.  If  the 
two  planes  should  turn  out  not  to  coincide,  it  will  open 
up  some  new  fields  in  celestial  mechanics.  The  belts 
have  been  thought  to  indicate  divergence,  but  the  most 
recent  observations  by  Perrotin  on  them  minimize  this. 
They  suggest,  too,  a  rotation  period  of  about  ten  hours, 
which  is  what  we  should  expect. 

Its  albedo,  or  intrinsic  brightness,  is,  according  to 
Miiller,  0.73,  or  almost  exactly  that  of  cloud.  This 
tallies  with  the  lack  of  pronouncement  of  the  belts  and 
is  another  argument  against  the  reality  of  the  recent 
diametral  measurements,  as  all  Miiller's  values  are 
got  by  dividing  the  amount  of  light  received  by  the 
amount  of  surface  sending  it.  If  the  diameter  were 


THE    OUTER    PLANETS  117 

much  less  than  thirty-two  thousand  miles,  the  result- 
ing albedo  would  become  impossibly  high. 

If  we  know  but  little  about  the  actual  surface  of 
Uranus,  we  know  now  a  good  deal  about  its  atmosphere. 
And  this  partly  because  atmosphere  is  almost  all  that  it 
is.  The  satellites  are  the  only  solid  thing  in  the  system. 
If  we  needed  a  telltale  that  the  solar  system  had  evolved, 
the  gaseous  constitution  of  its  primaries  and  the  con- 
densed state  of  their  attendants  would  sufficiently  in- 
form us.  Probably  all  the  major  planets  are  nothing 
but  gas.  It  has  been  debated  whether  Jupiter  be  al- 
most all  vapor  with  a  solid  kernel  beneath,  or  vapor 
entirely.  That  he  grows  denser  toward  the  core  is 
doubtless  the  case,  but  that  he  is  anywhere  other  than 
a  gaseous  fluid  is  very  unlikely.  For  if  he  had  really 
begun  to  condense,  he  must  have  contracted  to  far 
within  his  present  dimensions.  The  same  is  true  of 
Uranus. 

The  surprising  thing  about  Uranus  is  the  enor- 
mous extent  of  his  atmosphere.  The  earliest  spectro- 
scopists  perceived  this,  but  the  more  spectroscopy 
advances,  the  greater  and  more  interesting  it  proves 
to  be.  By  pushing  inquiry  into  the  red  end  of  the 
spectrum,  hitherto  a  terra  incognita,  Dr.  Slipher  has 
uncovered  a  mass  of  as  yet  unexplained  revelation. 
Of  these  remarkable  spectrograms  we  shall  speak 
later.  Here  it  is  sufficient  to  say  that  so  great  is  the 


n8        THE   EVOLUTION   OF   WORLDS 

absorption  in  the  red  that  only  the  blue  and  green  in 
anything  like  their  entirety  get  through;  which  ac- 
counts for  the  well-known  sea-green  look  of  the  planet. 
Furthermore,- the  spectroscope  shows  that  this  atmos- 
phere, or  the  great  bulk  of  it,  must  lie  above  what  we 
see  as  the  contour  of  the  disk.  For  the  spectroscope 
is  as  incapable  of  seeing  through  opacity  as  the  eye, 
though  it  distances  the  eye  in  seeing  the  invisible.  It 
is  not  what  is  condensed  into  cloud,  but  what  is  not, 
of  which  it  reveals  the  presence.  We  are  thus  made 
aware  of  a  great  shell  of  air  enveloping  the  planet. 

In  Uranus,  then,  we  see  a  body  in  an  early  amor- 
phous state,  before  the  solid,  the  liquid,  and  the  gaseous 
conditions  of  matter  have  become  differentiate  and 
settled  each  into  distinctive  place.  Without  even  an 
embryo  core  its  substance  passes  from  viscosity  to  cloud. 

Neptune  has  proved  a  planet  of  surprises.  Though 
its  orbital  revolution  is  performed  direct,  its  rotation 
apparently  takes  place  backward,  in  a  plane  tilted 
about  35°  to  its  orbital  course.  Its  satellite  certainly 
travels  in  this  retrograde  manner.  Then  its  appearance 
is  unexpectedly  bright,  while  its  spectrum  shows  bands 
which  as  yet,  for  the  most  part,  defy  explanation,  though 
they  state  positively  the  vast  amount  of  its  atmosphere 
and  its  very  peculiar  constitution.  But  first  and  not 
least  of  its  surprises  was  its  discovery, —  a  set  of  sur- 
prises, in  fact.  For  after  owing  recognition  to  one  of 


THE   OUTER   PLANETS  119 

the  most  brilliant  mathematical  triumphs,  it  turned 
out  not  to  be  the  planet  expected. 

"  Neptune  is  much  nearer  the  Sun  than  it  ought  to 
be,"  is  the  authoritative  way  in  which  a  popular  histo- 
rian puts  the  intruding  planet  in  its  place.  For  the 
planet  failed  to  justify  theory  by  not  fulfilling  Bode's 
law,  which  Leverrier  and  Adams,  in  pointing  out  the  dis- 
turber of  Uranus,  assumed  "  as  they  could  do  no  other- 
wise." Though  not  strictly  correct,  as  not  only  did 
both  geometers  do  otherwise,  but  neither  did  otherwise 
enough,  the  quotation  may  serve  to  bring  Bode's  law 
into  court,  as  it  was  at  the  bottom  of  one  of  the  strangest 
and  most  generally  misunderstood  chapters  in  celestial 
mechanics. 

Very  soon  after  Uranus  was  recognized  as  a  planet, 
approximate  ephemerides  of  its  motion  resulted  in 
showing  that  it  had  several  times  previously  been 
recorded  as  a  fixed  star.  Bode  himself  discovered 
the  first  of  these  records,  one  by  Mayer  in  1756,  and 
Bode  and  others  found  another  made  by  Flamstead 
in  1690.  These  observations  enabled  an  elliptic  orbit 
to  be  calculated  which  satisfied  them  all.  Subse- 
quently others  were  detected.  Lemonnier  discovered 
that  he  had  himself  not  discovered  it  several  times, 
cataloguing  it  as  a  fixed  star.  Flamstead  was  spared  a 
like  mortification  by  being  dead.  For  both  these  ob- 
servers had  recorded  it  two  or  more  nights  running, 


120        THE   EVOLUTION   OF   WORLDS 

from  which  it  would  seem  almost  incredible  not  to 
have  suspected  its  character  from  its  change  of  place. 

Sixteen  of  these  pre-discovery  observations  were  found 
(there  are  now  nineteen  known),  which  with  those 
made  upon  it  since  gave  a  series  running  back  a  hun- 
dred and  thirty  years,  when  Alexis  Bouvard  prepared 
his  tables  of  the  planet,  the  best  up  to  that  time,  pub- 
lished in  1821.  In  doing  so,  however,  he  stated  that 
he  had  been  unable  to  find  any  orbit  which  would 
satisfy  both  the  new  and  the  old  observations.  He 
therefore  rejected  the  old  as  untrustworthy,  forgetting 
that  they  had  been  satisfied  thirty  years  before,  and 
based  his  tables  solely  on  the  new,  leaving  it  to  pos- 
terity, he  said,  to  decide  whether  the  old  observations 
were  faulty  or  whether  some  unknown  influence  had 
acted  on  the  planet.  He  had  hardly  made  this  invidi- 
ous distinction  against  the  accuracy  of  the  ancient 
observers  when  his  own  tables  began  to  be  out  and 
grew  seriously  more  so,  so  that  within  eleven  years 
they  quite  failed  to  represent  the  planet. 

The  discrepancies  between  theory  and  observation 
attracted  the  attention  of  the  astronomic  world,  and  the 
idea  of  another  planet  began  to  be  in  the  air.  The  great 
Bessel  was  the  first  to  state  definitely  his  conviction  in 
a  popular  lecture  at  Konigsberg  in  1840,  and  thereupon 
encouraged  his  talented  assistant  Flemming  to  be- 
gin reductions  looking  to  its  locating.  Unfortunately, 


THE   OUTER    PLANETS  121 

in  the  midst  of  his  labors  Flemming  died,  and  shortly 
after  Bessel  himself,  who  had  taken  up  the  matter  after 
Flemming's  death. 

Somewhat  later  Arago,  then  head  of  the  Paris  observ- 
atory, who  had  also  been  impressed  with  the  existence 
of  such  a  planet,  requested  one  of  his  assistants,  a  re- 
markable young  mathematician  named  Leverrier,  to  un- 
dertake its  investigation.  Leverrier,  who  had  already 
evidenced  his  marked  ability  in  celestial  mechanics,  pro- 
ceeded to  grapple  with  the  problem  in  the  most  thor- 
ough manner.  He  began  by  looking  into  the  pertur- 
bations of  Uranus  by  Jupiter  and  Saturn.  He  started 
with  Bouvard's  work,  with  the  result  of  finding  it  very 
much  the  reverse  of  good.  The  farther  he  went,  the 
more  errors  he  found,  until  he  was  obliged  to  cast  it 
aside  entirely  and  recompute  these  perturbations  himself. 
The  catalogue  of  Bouvard's  errors  he  gave  must  have 
been  an  eye-opener  generally,  and  it  speaks  for  the 
ability  and  precision  with  which  Leverrier  conducted 
his  investigation  that  neither  Airy,  Bessel,  nor  Adams 
had  detected  these  errors,  with  the  exception  of  one  term 
noticed  by  Bessel  and  subsequently  by  Adams.*  The 
result  of  this  recalculation  of  his  was  to  show  the  more 
clearly  that  the  irregularities  in  the  motion  of  Uranus 
could  not  be  explained  except  by  the  existence  of  an- 
other planet  exterior  to  him.  He  next  set  himself  to 

*  Adams,  "Explanation  of  the  Motion  of  Uranus,"  1846. 


122        THE   EVOLUTION   OF   WORLDS 

locate  this  body.  Influenced  by  Bode's  law,  he  began 
by  assuming  it  to  lie  at  twice  Uranus'  distance  from 
the  Sun,  and,  expressing  the  observed  discrepancies  in 
longitude  in  equations,  comprising  the  perturbations 
and  possible  errors  in  the  elements  of  Uranus,  proceeded 
to  solve  them.  He  could  get  no  rational  solution.  He 
then  gave  the  distance  and  the  extreme  observations 
a  certain  elasticity,  and  by  this  means  was  able  to 
find  a  position  for  the  disturber  which  sufficiently  satis- 
fied the  conditions  of  the  problem.  Leverrier's  first 
memoir  on  the  subject  was  presented  to  the  French 
Academy  on  November  10,  1845,  that  giving  the  place 
of  the  disturbing  planet  on  June  i,  1846.  There  is  no 
evidence  that  the  slightest  search  in  consequence  was 
made  by  anybody,  with  the  possible  exception  of  the 
Naval  Observatory  at  Washington.  On  August  31 
he  presented  his  third  paper,  giving  an  orbit,  mass, 
and  more  precise  place  for  the  unknown.  Still  no 
search  followed.  Taking  advantage  of  the  acknowl- 
edging of  a  memoir,  Leverrier,  in  September,  wrote  to 
Dr.  Galle  in  Berlin  asking  him  to  look  for  the  planet. 
The  letter  reached  Galle  on  the  23d,  and  that  very  night 
he  found  a  planet  showing  a  disk  just  as  Leverrier  had 
foretold,  and  within  55'  of  its  predicted  place. 

The  planet  had  scarcely  been  found  when,  on  October 
I,  a  letter  from  Sir  John  Herschel  appeared  in  the  Lon- 
don Athenaum  announcing  that  a  young  Cambridge 


THE   OUTER    PLANETS  123 

graduate,  Mr.  J.  C.  Adams,  had  been  engaged  on  the 
same  investigation  as  Leverrier,  and  with  similar  results. 
This  was  the  first  public  announcement  of  Mr.  Adams' 
labors.  It  then  appeared  that  he  had  started  as  early 
as  1843,  and  had  communicated  his  results  to  Airy  in 
October,  1 845,  a  year  before.  Into  the  sad  set  of  circum- 
stances which  prevented  the  brilliant  young  mathema- 
tician from  reaping  the  fruit  of  what  might  have  been 
his  discovery,  we  need  not  go.  It  reflected  no  credit  on 
any  one  concerned  except  Adams,  who  throughout  his 
life  maintained  a  dignified  silence.  Suffice  it  to  say 
that  Adams  had  found  a  place  for  the  unknown  within 
a  few  degrees  of  Leverrier's;  that  he  had  communi- 
cated these  results  to  Airy;  that  Airy  had  not  consid- 
ered them  significant  until  Leverrier  had  published  an 
almost  identical  place;  that  then  Challis,  the  head  of 
the  Cambridge  Observatory,  had  set  to  work  to  search 
for  the  planet  but  so  routinely  that  he  had  actually 
mapped  it  several  times  without  finding  that  he  had 
done  so,  when  word  arrived  of  its  discovery  by  Galle. 
But  now  came  an  even  more  interesting  chapter  in 
this  whole  strange  story.  Mr.  Walker  at  Washington 
and  Dr.  Petersen  of  Altona  independently  came  to 
the  conclusion  from  a  provisional  circular  orbit  for  the 
newcomer  that  Lalande  had  catalogued  in  the  vicinity 
of  its  path.  They  therefore  set  to  work  to  find  out 
if  any  Lalande*  stars  were  missing.  Dr.  Petersen 


i24        THE   EVOLUTION  OF   WORLDS 

compared  a  chart  directly  with  the  heavens  to  the 
finding  a  star  absent,  which  his  calculations  showed 
was  about  where  Neptune  should  have  been  at  the 
time.  Walker  found  that  Lalande  could  only  have 
swept  in  the  neighborhood  of  Neptune  on  the  8th  and 
loth  of  May,  1795.  By  assuming  different  eccentric- 
ities for  Neptune's  orbit  under  two  hypotheses  for 
the  place  of  its  perihelion,  he  found  a  star  catalogued 
on  the  latter  date  which  sufficiently  satisfied  his  com- 
putations. He  predicted  that  on  searching  the  sky  this 
star  would  be  found  missing.  On  the  next  fine  even- 
ing Professor  Hubbard  looked  for  it,  and  the  star  was 
gone.  It  had  been  Neptune.* 

This  discovery  enabled  elliptic  elements  to  be  com- 
puted for  it,  when  the  surprising  fact  appeared  that 
it  was  not  moving  in  anything  approaching  the  orbit 
either  Leverrier  or  Adams  had  assigned.  Instead  of  a 
mean  distance  of  36  astronomical  units  or  more,  the 
stranger  was  only  at  30.  The  result  so  disconcerted 
Leverrier  that  he  declared  that  "the  small  eccentricity 
which  appeared  to  result  from  Mr.  Walker's  compu- 
tations would  be  incompatible  with  the  nature  of  the 
perturbations  of  the  planet  Herschel,"  as  he  called 
Uranus.  In  other  words,  he  expressly  denied  that 
Neptune  was  his  planet.  For  the  newcomer  pro- 
ceeded to  follow  the  path  Walker  had  computed.  This 

*  Proc.  Amer.  Acad.,  Vol.  I,  p.  64. 


THE   OUTER   PLANETS  125 

was  strikingly  confirmed  by  Mauvais'  discovering  that 
Lalande  had  observed  the  star  on  the  8th  of  May  as 
well  as  on  the  loth,  but  because  the  two  places  did 
not  agree,  he  had  rejected  the  first  observation,  and 
marked  the  second  as  doubtful,  thus  carefully  avoiding 
a  discovery  that  actually  knocked  at  his  door. 

Meanwhile  Peirce  had  made  a  remarkable  contri- 
bution to  the  whole  subject.  In  a  series  of  profound 
papers  presented  to  the  American  Academy,  he  went 
into  the  matter  more  generally  than  either  of  the 
discoverers,  to  the  startling  conclusion  "that  the  planet 
Neptune  is  not  the  planet  to  which  geometrical  analysis 
had  directed  the  telescope,  and  that  its  discovery  by 
Galle  must  be  regarded  as  a  happy  accident."  *  He 
proved  this  first  by  showing  that  Leverrier's  two 
fundamental  propositions,  — 

1.  That    the    disturber's    mean    distance    must    be 
between  35  and  37.9  astronomical  units; 

2.  That  its  mean  longitude  for  January  I,  1800,  must 
have  been  between  243°  and  252°, — 

were  incompatible  with  Neptune.  Either  alone  might 
be  reconciled  with  the  observations,  but  not  both. 

In  justification  of  his  assertion  that  the  discovery 
was  a  happy  accident,  he  showed  that  three  solutions 
of  the  problem  Leverrier  had  set  himself  were  possible, 
all  equally  complete  and  decidedly  different  from 

*  Proc.  Amer.  Acad.,  Vol.  I,  p.  65  et  seq. 


126        THE   EVOLUTION   OF  WORLDS 

each  other,  the  positions  of  the  supposed  planet  being 
120°  apart.  Had  Leverrier  and  Adams  fallen  upon 
either  of  the  other  two,  Neptune  would  not  have  been 
discovered.* 

He  next  showed  that  at  35.3  astronomical  units,  an 
important  change  takes  place  in  the  character  of  the 
perturbations  because  of  the  commensurability  of  period 
of  a  planet  revolving  there  with  that  of  Uranus.  In  con- 
sequence of  which,  a  planet  inside  of  this  limit  might 
equally  account  for  the  observed  perturbations  with  the 
one  outside  of  it  supposed  by  Leverrier.  This  Nep- 
tune actually  did.  From  not  considering  wide  enough 
limits,  Leverrier  had  found  one  solution,  Neptune  ful- 
filled the  other.f  And  Bode's  law  was  responsible  for 
this.  Had  Bode's  law  not  been  taken  originally  as 
basis  for  the  disturber's  distance,  those  two  great 
geometers,  Leverrier  and  Adams,  might  have  looked 
inside. 

This  more  general  solution,  as  Peirce  was  careful 
to  state,  does  not  detract  from  the  honor  due  either 
to  Leverrier  or  to  Adams.  Their  masterly  calculations, 
the  difficulty  of  which  no  one  who  has  not  had  some 
experience  of  the  subject  can  appreciate,  remain  as  an 
imperishable  monument  to  both,  as  does  also  Peirce's 
to  him. 

*  Proc.  Amer.  Acad.,  Vol.  I,  p.  144. 
f  Proc.  Amer.  Acad.,  Vol.  I,  p.  332. 


CHAPTER   V 

FORMATION    OF    PLANETS 

IN  our  first  two  chapters  we  saw  what  sign-posts 
in  the  sky  there  are  pointing  to  the  course  evo- 
lution of  a  solar  system  probably  follows,  and  sec- 
ondly, what  evidence  there  is  that  our  system  took 
this  road.  We  now  come  to  a  question  not  so  easy 
to  precise,  —  the  actual  details  of  the  journey.  It  is 
always  difficult  to  descend  from  a  glittering  panoramic 
survey  to  particular  path-finding.  The  obstacles  loom 
so  much  larger  on  a  near  approach. 

Most  men  shy  at  decisions  and  shun  self-committal 
to  any  positive  course,  but  when  it  comes  to  constructing 
a  cosmogony,  few  at  all  qualified  hesitate  to  frame  one 
if  the  old  does  not  suit.  The  safety  in  so  doing  lies  in 
the  fact  that  nothing  in  particular  happens  if  it  re- 
fuses to  work.  Its  absurdity  is  promptly  shown  up,  it 
is  true,  by  some  one  else.  For  there  is  almost  as  good 
a  trade  in  exposing  cosmogonies  as  in  constructing 
them.  But  no  special  opprobrium  attaches  to  failure, 
because  everybody  has  failed,  from  Laplace  down,  or 
up,  as  you  are  pleased  to  consider  it.  Besides  it  is 
really  not  so  easy  to  do,  as  one  is  tempted  to  believe 

127 


i28        THE   EVOLUTION   OF  WORLDS 

before  his  book  is  published.  Then  only  does  the 
difficulty  dawn,  with  a  speed  and  clarity  inversely 
proportional  to  the  previous  relation  of  the  critic  to  the 
author.  For  the  author  himself  is  apt  to  be  blind. 
With  the  fatal  fondness  of  a  parent  for  his  offspring  it 
is  rare  for  the  defects  to  be  so  glaringly  apparent  to 
their  perpetrator.  At  the  worst  he  considers  them 
venial  faults  which  can  be  glossed  away. 

Attacking  the  subject  in  this  judicial  spirit,  the  reader 
can  hardly  expect  me  to  satisfy  him  with  a  cosmogony 
entirely  home-made,  but  at  best  to  pursue  a  happy  middle 
course  between  creator  and  critic,  advocating  only  such 
portions  as  happen  to  be  my  own,  while  sternly  exposing 
the  mistakes  of  others. 

In  undertaking  the  hazardous  climb  toward  the  origin 
of  things  two  qualities  are  necessary  in  the  explorer : 
a  quick  eye  for  possibilities  and  a  steady  head  in  testing 
them.  Without  the  discernment  to  perceive  relations 
no  ascent  to  first  principles  is  possible;  and  without 
the  support  of  quantitative  criterion,  one  is  in  danger 
of  becoming  giddy  from  one's  own  imagination.  Con- 
gruities  must  first  hint  at  a  path;  physical  laws  then 
determine  its  feasibility. 

An  eye  for  congruities  is  the  first  essential.  For 
congruity  alone  accuses  an  underlying  law.  It  is  the 
analogic  that  with  logic  leads  to  great  generalizations. 
Certain  concords  of  the  sort  in  the  motions  of  the  planets 


FORMATION   OF   PLANETS  129 

were  what  suggested  to  Laplace  his  system  of  the 
world.  With  the  uncommon  sense  of  a  mathematician 
he  perceived  that  such  accordances  were  not  necessitated 
by  the  law  of  gravitation,  and  on  the  other  hand,  could 
not  be  due  to  chance.  The  laws  of  probability  showed 
millions  to  one  against  it.  One  of  these  happy  harmon- 
ies was  that  all  the  large  planets  revolved  about  the  Sun 
in  substantially  the  same  plane;  another  that  they  all 
travelled  in  the  same  sense  (direction).  Had  they  been 
unrelated  bodies  at  the  start,  such  agreement  in  motion 
was  mathematically  impossible.  Their  present  con- 
sensus implied  a  common  origin  for  all.  In  other 
words,  the  solar  system  must  have  grown  to  be  what  it 
is,  not  started  so. 

This  basic  fact  we  may  consider  certain.  But  from 
it  we  would  fain  go  on  to  find  out  how  it  evolved.  To 
do  so  the  same  process  must  be  followed.  Considering, 
then,  our  solar  system  from  this  point  of  view,  one  can- 
not but  be  struck  by  some  further  congruities  it  presents. 
These  are  not  quite  those  that  inspired  Laplace,  because 
of  discoveries  since,  and  demand  in  consequence  a  theory 
different  from  his. 

The  out  about  constructing  a  theory  is  that  fresh 
facts  will  come  along  and  knock  for  admission  after 
the  door  is  shut.  They  prove  irreconcilables  because 
they  were  not  consulted  in  advance.  The  conse- 
quence is  that  since  Laplace's  time  new  relations  have 


130        THE   EVOLUTION   OF   WORLDS 

come  to  light,  and  some  supposed  concords  have  had  to 
be  given  up;  so  that  were  he  alive  to-day  he  would  him- 
self have  formulated  some  other  scheme.  Two,  how- 
ever, are  still  as  true :  that  the  planets  all  revolve  in  the 
same  plane  and  in  the  same  sense,  and  that  sense  that 
of  the  Sun's  rotation.  But  so  general  a  congruity  as 
this  points  only  to  an  original  common  moment  of 
momentum  and  is  equally  explicable  however  that 
motion  was  brought  about.  It  seems  quite  compatible 
with  an  original  shock.  To  say  that  it  was  caused  by  a 
disruption  is  simply  to  go  one  step  farther  back  than  La- 
place. If,  then,  such  a  catastrophe  did  occur  as  the 
meteorites  aver,  we  may  perhaps  draw  some  interesting 
inferences  about  it  from  the  present  state  of  the  system. 
In  a  very  close  approach  such  as  we  must  suppose  for 
the  disruption,  one  within  Roches'  limit  of  2.5  diame- 
ters, the  stranger,  supposing  him  of  equal  size,  would 
sweep  from  one  side  of  the  former  Sun  to  the  other  in 
about  two  hours,  and  the  brunt  of  the  disrupting  pull 
occur  within  that  time.  That  the  former  Sun  was  ro- 
tating slowly  seems  established  by  the  time,  twenty- 
eight  days,  it  now  takes  to  go  round.  In  which  case 
the  orbits  of  the  masses  which  were  to  form  the  planets 
would  all  lie  in  about  the  same  plane, —  the  plane  of 
the  tramp's  approach.  If  there  were  exceptions,  they 
should  be  found  in  the  innermost.  For  such  should 
partake  most  largely  of  the  Sun's  own  original  rota- 


FORMATION   OF   PLANETS  131 

tion  and  travel  therefore  most  nearly  in  its  plane. 
And  as  a  fact  Mercury,  the  Benjamin,  does  differ  from 
the  others  by  revolving  in  a  plane  inclined  some  7°  to 
their  mean,  agreeing  in  this  with  the  Sun's  own  rota- 
tion, with  whose  plane  it  was  probably  originally  coin- 
cident (digression  from  it  now  being  due  to  secular 
retrogression  of  the  planets'  nodes).4 

From  the  relations  which  advance  has  left  unchanged 
we  pass  to  those  phenomena  which  seemed  to  present 
congruities  in  Laplace's  day,  but  which  have  since 
proved  void  owing  to  subsequent  detection  of  excep- 
tions. Time  prevents  my  making  the  catalogue  com- 
plete, but  the  reader  shall  be  shown  enough  to  satisfy 
him  of  the  problem's  complexity  and  to  whet  his  desire 
for  further  research  —  on  the  part,  preferably,  of  others. 

First  comes,  then,  the  rotations  of  the  planets  upon 
their  axes,  which  Laplace  supposed  to  be  all  in  the  same 


1453 


SUN  JUPITER      SATURN  URANUS  NEPTUNE 

CHART  SHOWING  INCREASING  TILTS  OF  THE  MAJOR  PLANETS. 

direction,  counter  to  the  hands  of  a  clock;  for  the  heav- 
ens mark  time  oppositely  from  us.  All  those  within 
and  including  Saturn,  the  only  ones  he  knew,  turn,  in- 
deed, in  the  same  sense  that  they  travel  round  the  Sun. 
But  Uranus  departs  from  that  direction  by  a  right  angle, 
wallowing  rather  than  spinning  in  his  orbit;  while  Nep- 


1 32        THE   EVOLUTION  OF   WORLDS 

tune  goes  still  farther  in  idiosyncratic  departure  and 
actually  turns  in  the  opposite  direction.  Here,  then, 
Laplace's  congruity  breaks  down,  but  in  its  place  a  little 
attention  will  show  that  a  new  one  has  arisen.  For 
Saturn's  tilt  is  27°  and  Jupiter's  3°,  so  that  with  the 
major  planets  there  is  revealed  a  systematic  righting 
of  the  planetary  axes  from  inversion  through  perpen- 
dicularity to  directness  as  one  proceeds  inward  toward 
the  Sun. 

Another  congruity  supposed  to  exist  a  century  ago 
was  the  exemplary  agreement  of  all  the  satellites  to  fol- 
low in  their  planetary  circuits  the  pattern  set  them  by 
their  primaries  round  the  Sun.  But  as  man  has  pene- 
trated farther  into  space  and  photographic  plates  have 
come  to  be  employed,  satellites  have  been  revealed 
which  depart  from  this  orderly  arrangement.  This  is 
the  case  with  the  ninth,  the  outermost,  satellite  of 
Saturn  and  with  the  eighth,  the  outermost,  of  Jupiter. 
But,  as  before,  the  breaking  down  of  one  congruity 
seems  but  the  establishing  of  another.  It  appears  that 
only  the  most  distant  satellites  are  permitted  such  un- 
conformity of  demeanor.  For  departure  from  the 
supposed  orthodoxy  occurs  in  both  instances  where  the. 
distance  is  most,  and  does  not  occur  in  the  case  of  all 
the  other  satellites  found  since  Laplace's  day,  eleven 
in  number,  nearer  their  planets. 

A  third  congruity  formerly  believed  in  has  suffered 


FORMATION   OF   PLANETS 


a  like  fate;  to  wit,  that  satellites  always  moved  in  or 
near  the  equatorial  plane  of  their  primary.  All  those 
first  discovered  did ;  the  four  large  ones  of  Jupiter,  the 
main  ones  of  Saturn,  and  probably  those  of  Uranus  and 


SATELLITE 

ECCENTRICITIES 
(ORBITAL) 


JUPITER          / 


DIRECT. 


SATURN 


s 

VIII 


JUPITER  /VH 

-  - 


SATURN 


Neptune.  Even  the  satellites  of  Mars  conformed, 
lapetus  alone  seemed  to  make  exception,  and  that  by  a 
glossable  amount.  But  this  orderliness,  too,  has  been 
disposed  of,  only,  like  the  others,  to  experience  a  res- 
urrection in  a  different  form. 

On  examining  more  precisely  the  inclinations  of 
these  orbits  some  years  ago,  an  interesting  relation 
between  them  and  the  distances  of  the  satellites  from 
their  primaries  forced  itself  on  my  notice.  The  tilt 


134        THE   EVOLUTION  OF  WORLDS 

increased  as  the  distance  grew.  The  only  exceptions 
were  very  tiny  bodies  occupying  a  sort  of  asteroidal 
relation  to  the  rest. 

A  diagram  will  make  this  clear.  The  kernel  of  it 
dates  from  the  lectures  then  delivered  before  the  Massa- 
chusetts Institute  of  Technology  in  1901.  The  inter- 
esting thing  now  about  it  is  that  the  congruity  there 
pointed  out  has  been  conformed  to  by  every  satellite 
discovered  since,  —  the  sixth,  seventh,  and  eighth  of 
Jupiter  and  the  ninth  and  tenth  of  Saturn.  It  is  evi- 
dent that  we  already  know  enough  of  the  geniture  of 
our  system  to  prophesy  something  about  it  and  have 
the  prophecy  come  true. 

Closely  connected  with  the  previous  relation  is  a 
fourth  concordance  clearly  of  mechanical  origin,  the 
relation  of  the  orbital  eccentricities  of  the  satellites  to 
their  distances  from  their  respective  planets.  The  satel- 
lites pursue  more  and  more  eccentric  orbits  according 
as  they  stand  removed  from  planetary  proximity. 

A  fifth  congruity  is  no  less  striking.  All  the  sat- 
ellites of  all  the  planets  that  we  can  observe  well  enough 
to  judge  of  turn  the  same  face  always  to  their  lords. 
That  the  Moon  does  so  to  the  Earth  is  a  fact  of  every- 
day knowledge,  and  the  telescope  hints  that  the  same 
respectful  regard  is  paid  by  Jupiter's  and  Saturn's 
retinues  to  them.  What  is  still  more  remarkable, 
Mercury  and  Venus  turn  out  to  observe  the  like  vassal 


FORMATION   OF   PLANETS  135 

etiquette  with  reference  to  the  Sun.  And  it  will  be 
noticed  that  they  stand  to  him  the  nearest  of  his  court. 
Here,  then,  is  a  law  of  proximity  which  points  conclu- 
sively to  some  well-established  force. 

Last  is  a  remarkable  congruity  which  study  disclosed 
to  me  likewise  some  years  ago,  and  which  has  received 
corroboration  in  discoveries  since.  This  congruity  is  the 
peculiar  arrangement  of  the  masses  in  the  solar  system. 

Consider  first  the  way  in  which  the  several  planets, 
as  respects  size,  stand  ordered  in  distance  from  the 
sun.  Nearest  to  him  is  Mercury,  the  smallest  of  all 
the  principal  ones.  Venus  and  the  Earth  follow,  each 
larger  than  the  last;  then  comes  Mars,  of  distinctly  less 
bulk,  and  so  to  the  asteroids,  of  almost  none.  After 
this  the  mass  rises  again  to  its  maximum  in  Jupiter, 
and  then  subsequently  falls  through  Saturn  to  Uranus 
and  Neptune.  Here  we  mark  a  more  or  less  regular 
gradation  between  mass  and  position,  a  curve  in  which 
there  are  two  ups  and  downs,  the  outer  swell  being 
much  the  larger,  though  the  inner,  too,  is  sufficiently 
pronounced. 

Now  turn  to  Saturn  and  his  family,  which  is  the 
most  numerous  of  the  secondary  systems  and  that 
having  the  greatest  span.  Under  Saturn's  wing,  as  it 
were,  is  the  ring,  itself  a  congeries  of  tiny  satellites. 
Then  comes  Mimas,  the  smallest  of  the  principal  ones; 
then  Enceladus,  a  little  larger;  then  Tethys,  the  biggest 


136        THE   EVOLUTION   OF  WORLDS 


of  the  three.  Next  stands  Dione,  smaller  than  Tethys. 
Then  the  mass  increases  with  Rhea,  reaching  its  cul- 
mination in  Titan,  after  which  it  declines  once  more. 
Strangely  reproductive  this  of  the  curve  we  marked  in 


UPPER  CURVE, 

CF  DIAMETERS 

LOWER,   OF  MASS 


EACH  SET 
DRAWN  TO  SCALE 

BUT  SCALE 
DIFFERS  IN   EACH 


CURVES, 
OF  DIAMETERS 


JUPITER         SOLAR  SYSTEM 


DISTANCE  FROM  JUPITER 

SATURN'S  SYSTEM 


TOIX 


ERION 
7>7STANCE  FROM  SATURN 

TITAN IA  URANUS'  SYSTEM 

I OBERON 


ARIEL 
S 
UMBRIEL 


DISTANCE  FROM  URANUS 

MASSES  or  PLANETS  AND  SATELLITES. 


the  arrangement  of  the  planets  themselves,  even  to  the 
little  inner  rise  and  fall.' 

Striking  as  such  analogous  ordering  is,  it  is  not  all. 
For,  scanning  the  Jovian  system,  we  find  the  main  curve 
here  again;  Ganymede,  the  Jupiter  or  Titan  of  the 
system,  standing  in  the  same  medial  position  as  they. 
Lastly,  taking  up  Uranus  and  his  family  of  satellites, 
the  same  order  is  observable  there.  Titania,  the 
largest,  is  posted  in  the  centre. 

Thus  the  order  in  which  the  little  and  the  big  are 
placed  with  reference  to  their  controlling  orb  is  the  same 


FORMATION   OF   PLANETS  137 

in  the  solar  system  and  in  that  of  every  one  of  its  satel- 
lite families.  Method  here  is  unmistakable.  Nor  is 
it  easy  to  explain  unless  the  cause  in  all  was  like.  That 
the  rule  in  the  placing  of  the  planets  should  be  faith- 
fully observed  by  them  in  the  ordering  of  their  own 
domestic  retinues,  is  not  the  least  strange  feature  of 
the  arrangement.  It  argues  a  common  principle  for 
both.  Not  less  significant  is  the  secondary  hump  in 
their  distribution,  denoting  recrudescence  farther  in  of 
the  primary  procedure  shown  without. 

One  point  to  be  particularly  noticed  in  these  latter- 
day  congruities  is  that  they  are  not  simply  general  con- 
cords like  the  older  ones  —  the  fact  that  the  planets 
move  in  one  plane  or  in  the  same  sense  in  that  plane  — - 
but  detailed  placings,  ordered  according  to  the  dis- 
tances of  the  planets  from  the  Sun  or  of  the  satellites 
from  the  planets.  They  are  thus  not  simply  of  the 
combinative  but  of  the  permutative  order  of  probabili- 
ties, a  much  higher  one ;  in  other  words,  the  chance  that 
they  can  be  due  to  chance  is  multiplicately  small.  Thus 
just  as  these  analogies  are  by  so  much  more  remarkable, 
so  are  they  by  so  much  more  cogent.  They  tell  us  not 
only  of  an  evolution,  but  they  speak  of  the  very  man- 
ner of  its  work.  They  do  not  simply  generalize,  they 
specify  the  mode  of  action.  The  difficulty  is  to  under- 
stand their  language.  It  is  a  case  of  celestial  hiero- 
glyphics to  which  we  lack  the  key. 


138        THE   EVOLUTION   OF   WORLDS 

In  attempting  now  to  discover  how  all  this  came 
about  we  notice  first  that  the  system  could  not  have 
originated  in  the  beautifully  simple  way  suggested  by 
Laplace,  because  of  several  impossibilities  in  the  path. 
If  rings  were  shed,  as  he  supposed,  from  a  symmetric 
contracting  mass,  they  should  have  resulted  in  some- 
thing even  more  symmetric  than  we  observe  to-day. 
In  the  next  place  they  could  not,  it  would  appear,  even 
if  formed,  have  collected  into  planets. 

Nor  could  there  have  been  an  original  "  fire-mist " 
with  which  as  a  stock  in  trade  Laplace  thriftily  endowed 
his  nebula  to  start  with  —  the  necessity  for  which  has 
been  likened  to  our  supposed  descent  from  monkeys; 
but  which  in  truth  is  as  misty  a  conception  of  the  facts 
in  the  one  case  as  it  is  a  monkeying  with  them  in  the 
other.  Darwin's  theory  distinctly  avers  that  we  were  not 
descended  from  monkeys;  and  Laplace's  fire-mist  under 
modern  examination  evaporates  away.  It  is  an  in- 
teresting outcome  of  modern  analysis  that  the  very  fact 
which  suggested  the  annular  genesis  of  planets  to  La- 
place, the  rings  of  Saturn,  should  now  probably  be 
deemed  a  striking  instance  of  the  reverse.  Far  from 
its  being  an  exemplar  in  the  heavens  of  the  pristine  state 
of  the  solar  system,  we  may  now  see  in  it  a  shining  pat- 
tern of  how  the  devolution  of  bodies  comes  about.  For 
instead  of  typifying  an  unfortunate  set  of  particles  which 
untoward  circumstance  has  prevented  from  coalescing 


FORMATION   OF   PLANETS  139 

into  a  single  orb,  it  almost  certainly  represents  the  dis- 
traught state  to  which  a  once  more  compact  congeries 
of  them  has  been  brought  by  planetary  interference. 
For  to  just  such  fate  must  the  stresses  in  it  caused  by 
Saturn  have  eventually  led.  Disruption  inevitable  to 
such  a  group  the  observation  of  comets  demonstrates 
is  daily  taking  place.  When  a  comet  passes  round  the 
Sun  or  near  a  planet,  the  partitive  pulls  of  the  body 
tend  to  dismember  it,  and  the  same  is  a  fortiori  true  of 
matter  circulating  round  a  planet  as  relatively  near  as 
the  meteoric  particles  that  constitute  Saturn's  rings. 
Starting  as  a  congeries,  it  was  pulled  out  more  and  more 
into  a  ring  until  it  became  practically  even  throughout. 
And  the  very  action  that  produced  it  tends  to  keep  it  as 
surprisingly  regular  as  we  note  to-day. 

No,  the  planets  probably  were  otherwise  generated 
and  may  have  looked  in  their  earlier  stages  as  the  knots 
in  the  spiral  nebulae  do  to-day.  But  this  does  not  mean 
that  we  can  detail  the  process.5 

Taking  now  the  congruities  for  guide,  we  proceed  to 
see  what  they  affirm  or  negative.  Laplace,  when  he 
ventured  on  his  exposition  of  the  system  of  the  world, 
did  so  "with  the  mistrust  which  everything  which  is  not 
the  direct  outcome  of  observation  or  calculation  must 
inspire."  To  all  who  know  how  even  figures  can  lie 
this  caution  will  seem  well  timed.  The  best  we  can 
do  to  keep  our  heads  steady  is  to  lay  firm  hold  at  each 


i4o        THE   EVOLUTION  OF  WORLDS 

step  on  the  great  underlying  principles  of  physics.  One 
of  these  is  the  conservation  of  the  moment  of  momen- 
tum. This  expression  embodies  one  of  the  grandest 
generalizations  of  cosmic  mechanics.  The  very  phrase 
is  fittingly  sonorous,  with  something  of  that  religious 
sublimity  which  the  dear  old  lady  said  she  found  such 
a  consolation  in  the  biblical  word  Mesopotamia.  In- 
deed the  idea  is  grand  for  its  very  simplicity.  Mo- 
mentum means  the  quantity  of  motion  in  a  body.  It 
is  the  speed  into  the  number  of  particles  or  the  mass. 
Moment  of  momentum  denotes  the  rotatory  power  of 
it  round  an  axis.  Now  the  curious  and  interesting 
thing  about  this  quantity  is  that  it  can  neither  be  di- 
minished nor  increased.  It  is  an  abstraction  from 
which  nothing  can  be  abstracted  —  but  results.  It  is  the 
one  unalterable  thing  in  a  universe  of  change.  What 
it  was  in  the  beginning  in  a  system,  that  it  forever  re- 
mains. Because  of  this  unchangeableness  we  can  use  it 
very  effectively  for  purposes  of  deduction.  One  of 
these  is  in  connection  with  that  other  great  principle  of 
physics,  the  conservation  of  energy.  By  the  mutual 
action  of  particles  on  one  another,  by  contraction,  by 
tidal  pulls,  and  so  on,  some  energy  of  motion  is  con- 
stantly being  changed  into  heat  and  thus  dissipated 
away.  Energy  of  motion,  therefore,  is  slowly  being 
lost  to  the  system,  and  the  only  stable  state  for  the 
bodies  composing  it  is  when  their  energy  of  motion  has 


FORMATION   OF   PLANETS  141 

decreased  to  the  minimum  consistent  with  the  initial 
moment  of  momentum.  This  principle  we  shall  find 
very  fecund  in  its  application.  It  means  that  our  whole 
system  is  evolving  in  a  way  to  lessen  its  energy  of  motion 
while  keeping  its  quantity  of  motion  unchanged.  The 
universe  always  does  a  thing  with  the  least  possible 
expenditure  of  force  and  gets  rid  of  its  superfluous 
energy  by  parting  with  it  to  space.  Philosophers  may 
wrangle  over  its  being  the  best  possible  of  worlds,  but 
it  is  incontrovertibly  mechanically  the  laziest,  which 
a  pessimistic  friend  of  mine  says  proves  it  the  best. 
Now  this  generalization  finds  immediate  use  in  ex- 
plaining certain  features  of  the  solar  system.  In  look- 
ing over  the  congruities  it  will  be  seen  that  deviation 
from  the  principal  plane  of  the  system  or  departure  from 
a  circular  orbit  is  always  associated  with  smallness  in 
size.  The  insignificant  bodies  are  the  erratic  ones. 
Now  it  has  been  shown  mathematically  in  several  dif- 
ferent ways  that  when  small  particles  collect  into  a  larger 
mass,  the  collisions  tend  to  make  the  resultant  orbit  of 
the  combination  both  more  circular  and  more  conform- 
ant to  the  general  plane  than  its  constituents.  But 
we  may  see  this  more  forthrightly  by  means  of  the 
general  principle  enunciated  above.  For  in  fact  both 
results  are  direct  outcomes  of  the  conservation  of  mo- 
ment of  momentum.  Given  a  certain  moment  of  mo- 
mentum for  the  system,  the  total  energy  of  the  bodies 


142        THE   EVOLUTION   OF   WORLDS 

is  least  when  they  all  move  in  one  plane.  This  is  evi- 
dent at  once  because  the  components  of  motion  at  right 
angles  to  the  principal  plane  add  nothing  to  the  mo- 
ment of  momentum  of  the  system.  It  is  also  least  when 
the  bodies  all  revolve  in  circles  about  the  centre  of 
gravity.  The  circle  has  some  interesting  properties 
which  almost  justify  the  regard  paid  to  it  by  the  ancients 
as  the  only  perfect  figure.  It  encloses  the  maximum 
area  for  a  given  periphery,  so  that  according  to  the 
old  legends,  if  one  were  given  as  much  land  as  he  could 
enclose  with  a  certain  bull's  hide,  he  should,  after  cutting 
the  hide  into  strips,  arrange  these  along  the  circumfer- 
ence of  a  circle.  Now  this  property  of  the  circle  is 
intimately  connected  with  the  fact  that  a  body  revolving 
in  a  circle  has  the  greatest  moment  of  momentum  for 
the  least  expenditure  of  energy.  For  under  the  same 
central  force  all  ellipses  of  the  same  longest  diameters  — 
major  axes  these  are  technically  called  —  are  described 
in  the  same  time,  and  with  the  same  energy,  and  of  all 
such,  the  circle  encloses  the  greatest  area,  which  area 
measures  the  moment  of  momentum.6 

Given  a  certain  moment  of  momentum,  then  the 
energy  is  least  when  the  bodies  all  move  in  one  plane 
and  all  travel  in  circles  in  that  plane.  As  energy  is  con- 
stantly being  dissipated  while  any  alteration  among 
the  bodies  is  going  on,  to  coplanarity  and  circularity  of 
path  all  the  bodies  must  tend,  if  by  collision  they  be  ag- 


FORMATION   OF   PLANETS  143 

gregated  into  larger  masses.  As  in  the  present  state  of 
our  system  the  small  bodies  travel  out  of  the  general 
plane  in  eccentric  ellipses  while  the  big  ones  travel  in  it 
in  approximate  circles,  the  facts  indicate  that  the  origin 
of  the  larger  masses  was  due  to  development  by  ag- 
gregation out  of  smaller  particles. 

The  next  principle  is  of  a  different  character.  Half 
a  century  ago  celestial  mechanics  dealt  with  bodies 
chiefly  as  points.  The  Earth  was  treated  as  a  weighted 
point,  and  so  was  the  Sun.  This  was  possible  because 
a  sphere  acts  upon  outside  bodies  as  if  all  its  mass 
were  collected  at  its  centre,  and  the  Sun  and  many 
of  the  planets  are  practically  spheres.  But  when  it 
came  to  nicer  questions  of  their  present  behavior  and 
especially  of  their  past  career,  it  grew  necessary  to  take 
their  shape  into  account  in  their  mutual  effects.  One 
of  the  results  was  the  discovery  of  the  great  role  played 
in  evolution  by  tidal  action.  Inasmuch  as  the  planets 
are  not  perfectly  rigid  bodies,  each  is  subject  to  tidal 
deformation  by  the  other,  the  outside  being  pulled 
more  than  the  centre  on  one  side  and  less  on  the  other. 
Bodily  tides  are  thus  raised  in  it  analogous  to  the  sur- 
face tides  we  see  in  the  ocean,  only  vastly  greater,  and 
these  in  turn  act  as  a  brake  on  its  rotation. 

Now  the  retrograde  motions  occurring  in  the  outer- 
most parts  of  all  the  systems,  principal  and  subsidiary, 
only  and  always  there:  the  retrograde  rotations  of 


i44        THE   EVOLUTION   OF   WORLDS 

Neptune  and  Uranus,  the  retrograde  revolutions  of  the 
ninth  satellite  of  Saturn  and  of  the  eighth  of  Jupiter, 
point  to  something  fundamental.  For  when  we  con- 
sider that  it  is  precisely  in  its  outer  portions  that  any 
forces  shaping  the  development  of  the  system  have  had 
less  time  to  produce  their  effect,  we  perceive  that  ap- 
parent abnormality  now  is  really  survival  of  the  original 
normal  state,  only  to  be  found  at  present  in  what  has  not 
been  sufficiently  forced  to  change.  It  suggests  that  the 
pristine  motion  of  the  constituents  of  the  scattered 
agglomerations  which  went  to  form  the  planets  was 
retrograde,  and  that  their  present  direct  rotations  and 
the  direct  revolutions  of  most  of  their  satellites  have 
been  imposed  by  some  force  acting  since.  Let  us  in- 
quire if  there  be  a  force  competent  to  this  end,  and 
what  its  mode  of  action. 

Let  us  see  how  tidal  action  would  work.  Tidal  force 
would  raise  bulges,  and  these,  not  being  carried  round 
with  the  planet's  rotation  except  to  a  certain  distance, 
due  to  viscosity,  must  necessarily  act  as  brakes  upon 
the  planet's  spin.  In  consequence  of  the  friction  they 
would  thus  exert,  energy  of  motion  must  be  lost.  So 
long,  then,  as  tidal  forces  can  come  into  play,  the  energy 
of  the  system  is  capable  of  decrease.  According  to  the 
last  principle  we  considered,  the  system  cannot  be  in 
stable  equilibrium  until  this  superfluous  energy  is  lost 
or  until  tidal  forces  become  inoperative,  which  cannot  be 


FORMATION   OF   PLANETS  145 

till  all  the  bodies  in  the  system  turn  the  same  face  to 
their  respective  centres  of  attraction. 

To  see  this  more  clearly,  take  the  case  of  a  retro- 
grade spin  of  a  planet  as  compared  with  a  direct  one. 
The  energy  of  the  planet's  spin  is  the  same  in  both  cases, 
because  energy  depends  on  the  square  of  a  quantity; 
to  wit,  that  of  the  velocity,  and  is  therefore  independent 
of  sign.  Not  so  the  moment  of  momentum.  For  this 
depends  on  the  first  power  of  the  speed,  and  if  positive 
in  the  one  case,  must  be  negative  in  the  other.  The 
moment  of  momentum  of  the  whole  system,  then,  is  less 
in  the  former  case,  since  the  moment  of  momentum  of 
the  retrograde  rotation  must  be  subtracted  from,  that 
of  the  direct  rotation  be  added  to,  that  of  the  rest  of  the 
system.  For  a  given  initial  moment  of  momentum 
with  which  the  system  was  endowed  at  the  start, 
there  is,  then,  superfluous  energy  in  the  first  state  which 
can  be  got  rid  of  through  reduction  to  the  second. 
Nature,  according  to  her  principles  of  least  exertion, 
avails  herself  of  the  chance  of  dispensing  with  it,  and 
a  direct  rotation  results.  Sir  Robert  Ball  first  sug- 
gested this  argument. 

Tidal  action  accomplishes  the  end.  In  checking  up 
a  body  rotating  contrary  to  the  general  consensus  of 
spin,  its  first  effect  is  to  start  to  turn  the  axis  over.  For 
the  body  is  in  dynamical  unstable  equilibrium  with 
regard  to  the  rest  of  the  system.  The  righting  would 


i46        THE   EVOLUTION   OF  WORLDS 

continue,  practically  to  the  exclusion  of  any  diminution 
at  first  of  the  spin,  until  the  body  had  turned  over  in  its 
plane  so  that  the  spin  became  direct.  As  the  force  in- 
creases greatly  with  nearness  to  the  Sun,  the  effect  would 
be  most  marked  on  the  nearer,  and  most  so  on  the  biggest, 
bodies.  This  would  account  for  the  otherwise  strange 
gradation  from  retrograde  to  direct  in  the  tilts  of  the 
axes  of  the  outer  planets,  and  also  for  the  present  tilts 
of  all  the  inner  ones. 

Related  to  the  initial  retrograde  rotations  of  the 
planets,  and  in  a  sense  survivals  from  an  earlier  state  of 
things,  are  two  of  the  latest  discoveries  of  motions  in  the 
solar  system,  the  retrograde  orbital  movements  of  the 
ninth  satellite  of  Saturn  and  the  eighth  of  Jupiter. 
Considered  so  anomalous  as  scarcely  at  first  to  be  be- 
lieved, it  has  been  stated  that  they  directly  contradict 
the  theory  of  Laplace.  This  is  true;  in  the  same 
sense  and  no  more  in  which  they  directly  contradict  the 
contradictor,  one  of  the  latest  theories.  For  neither 
theory  has  anything  to  explain  them  as  the  result  of  law. 
That  they  cannot  be  the  sport  of  indifferent  chance 
seems  evidenced  by  their  occupying  similar  external 
positions  in  their  respective  systems.  As  the  product 
of  a  law  we  must  regard  them,  and  to  find  that  law  we 
now  turn.  Suppose  the  planet  originally  to  have  been 
rotating  backward,  or  in  the  direction  of  the  hands  of  a 
clock.  At  this  time  the  satellite,  which  may  never  have 


FORMATION  OF   PLANETS  147 

formed  a  part  of  its  mass,  was  travelling  backward 
too,  according  to  what  we  have  said.  Then  under  the 
friction  of  the  tides  raised  on  the  planet  by  the  Sun,  the 
planet  proceeded  to  turn  over.  It  continued  to  do  so 
until  it  spun  direct.  During  this  process  there  was  no 
passage  through  zero  of  its  moment  of  momentum  con- 
sidered with  regard  to  itself,  and  therefore  no  difficulty 
on  that  score  of  supposing  that  it  successively  generated 
satellites  at  all  degrees  of  inclination.  That  its  children 
are  of  the  nature  of  adopted  waifs,  Babinet's  criterion 
(1861)  would  seem  to  imply.  But  it  must  be  remem- 
bered that  the  Sun  has  been  slowing  up  the  planet's 
rotation  now  for  aeons.  As  it  turned  over,  its  tidal 
bulges  tended  to  carry  over  with  it  such  satellites  as 
it  already  had.  This  effect  was  much  greater  on  the 
nearer  ones,  both  because  they  were  nearer  and  be- 
cause they  were  much  larger  than  the  outer.  So  that 
the  nearer  kept  with  the  planet,  the  others  lagged  pro- 
portionately behind.  This  suggests  itself  to  account  for 
the  facts,  but  the  subject  involves  so  much  that  is  un- 
certain that  I  submit  the  hypothesis  with  the  distrust 
which  Laplace  has  so  eminently  bespoken.  I  advance 
in  its  favor  only  the  three  striking  facts :  that  a  steady 
progression  in  their  tilts  of  rotation  is  observable  from 
Neptune  to  Jupiter  and  a  substantially  accordant  one 
from  Mars  to  Mercury;  secondly,  that  the  satellites 
turn  their  faces  to  their  primaries,  as  likewise  do  Mer- 


148        THE   EVOLUTION  OF   WORLDS 

cury  and  Venus  to  the  Sun;  and,  thirdly,  that  the  orbits 
of  the  satellites  of  all  the  planets  are  themselves  tilted 
in  accordance  with  what  it  would  require.7 

After  the  axial  spins  have  been  made  over  to  the  same 
sense,  the  second  consequence  of  tidal  action  in  the  case 
of  two  bodies  revolving  about  their  common  centre  of 
gravity  is  to  slow  down  both  spins  until  first  the  smaller 
and  then  the  larger  turn  the  same  face  to  each  other 
and  remain  thus  constant  ever  after.  Now  such  is 
precisely  the  pass  to  which  we  observe  the  satellites 
of  the  planets  have  come.  All  that  we  can  be  sure  of 
now  turn  the  same  face  always  to  their  primary.  The 
Moon  was  the  first  to  betray  her  attitude,  because  the 
one  we  can  best  note.  On  scrutiny,  however,  Jupi- 
ter's satellites,  so  far  as  we  can  make  out,  do  the  like; 
and  Saturn's,  too.  And  a  very  proper  attitude  it  is, 
this  regard  paid  to  compelling  attraction.  Thus  one  of 
the  congruities  we  noticed  stands  accounted  for.  The 
satellites  could  hardly  have  been  at  first  so  observant; 
time  has  brought  about  this  unfailing  recognition  of 
their  lords. 

Of  the  peculiar  massing  of  the  bodies  in  the  family 
of  the  Sun,  and  the  still  stranger  copying  of  it  in  their 
own  domestic  circles,  little  can  as  yet  be  said  in  in- 
terpretation. That  the  planetary  families  and  their 
ancestral  group  should  agree  is  not  the  least  strange  part 
of  the  affair.  It  shows  that  none  of  them  was  fortuitous, 


FORMATION   OF   PLANETS  149 

but  that  at  the  formation  of  all  some  common  principle 
presided,  apportioning  the  aggregations  to  their  proper 
place.  But  it  is  such  fine  print  of  the  system's  history 
as  at  present  to  preclude  discernment. 

So  much  for  the  details  we  may  deduce  of  the  method 
of  our  birth.  We  perceive  unmistakably  that  our  solar 
system  grew  to  be  what  it  is,  and  that  it  developed  by 
agglomeration  of  its  previously  shattered  fragments 
into  the  planets  we  behold  to-day,  but  exactly  how  the 
process  progressed  we  are  as  yet  unable  to  precise.  We 
are,  however,  as  what  I  have  mentioned  and  tabled 
show,  every  day  accumulating  data  which  will  enable 
an  eventual  determination  probably  to  be  reached. 

From  the  fact  of  agglomeration,  the  essence  of  the 
affair,  we  turn  to  the  traces  it  has  left  upon  its  several 
offspring. 

Just  as  the  continued  existence  to-day  of  meteorites 
in  statu  quo  informs  us  of  a  previous  body  from  which 
our  nebula  sprang;  so  a  physical  characteristic  of  our 
own  earth  at  the  present  time  shows  it  to  have  evolved 
from  that  nebula  —  even  though  we  cannot  make  out 
all  the  steps.  Of  its  having  done  so,  we  are  far  more 
sure  than  of  how  it  did. 

That  primitive  man  perceived  that  somewhere  below 
him  was  a  fiery  region  which  was  not  an  agreeable 
abode,  is  plain  from  his  consigning  to  such  Tophet 
those  whose  religious  tenets  did  not  square  with  his 


1 5o        THE   EVOLUTION   OF   WORLDS 

own.  That  his  conception  of  it  was  not  strictly  scien- 
tific is  evidenced  by  his  not  realizing  that  to  bury  his 
enemies  was  the  way  to  make  them  take  the  first  step  of 
the  journey  thither.  Indeed,  the  vindictive  venting  of 
his  notions  clearly  indicates  their  source  as  volcanic, 
rather  than  bred  of  a  general  disapproval  of  a  down- 
ward descent  either  in  silicates  or  sin. 

It  was  not  till  man  began  to  bore  into  the  Earth  for 
metallic  or  potable  purposes  that  he  brought  to  light 
the  generic  fact  that  it  was  everywhere  hotter  as  one 
went  down.  And  this  not  only  in  a  very  regular,  but  in 
a  most  speedy,  manner.  The  temperature  increased 
in  a  really  surprising  way  i°  F.  for  every  sixty-five  feet 
of  descent.  As  the  rise  continued  unabated  to  the 
limit  of  his  borings,  becoming  very  unpleasant  at  its 
end,  it  was  clear  that  at  a  depth  of  thirty-five  miles 
even  so  refractory  a  substance  as  platinum  must  melt, 
and  practically  all  the  Earth  except  a  thin  crust  be 
molten  or  even  gaseous. 

Now  heat,  like  money,  is  easy  to  dissipate  but  hard 
to  acquire,  as  primitive  man  was  the  first  to  realize.  It 
does  not  come  without  cause.  Being  a  mode  of  motion, 
other  motion  must  have  preceded  it  from  wrhich  it 
sprang.  So  much  the  doctrine  of  the  conservation  of 
energy  teaches  us,  a  doctrine  considered  now  to  have 
been  the  great  scientific  heirloom  of  the  nineteenth 
century  to  the  twentieth,  yet  which  in  its  day  caused  the 


FORMATION   OF   PLANETS  151 

death  of  its  first  discoverer,  Mayer,  of  a  broken  heart 
from  non-recognition;  its  second,  Helmholtz,  was 
refused  publication  by  the  leading  Berlin  physical 
magazine  of  the  time.  So  quick  is  man  to  delay  his 
own  advance. 

The  only  conceivable  motion  for  thus  heating  the 
Earth  as  a  whole  was  the  falling  together  of  its  parts. 
The  present  heat  of  the  Earth,  then,  accuses  the  con- 
course of  particles  in  the  past  to  its  formation,  or  in  other 
words  proves  that  the  Earth  was  evolved  out  of  material 
originally  more  sparcely  strewn.  It  does  so  not  only  in 
a  generic  but  in  a  most  particular  manner,  for  the  heat 
is  distributed  just  where  it  would  be  by  such  a  process. 
It  is  greater  to-day  within,  increasingly,  because  when 
the  globe  began  to  cool,  the  surface  necessarily  cooled 
first  and  established  a  regular  gradient  of  heat  from 
core  to  cuticle. 

It  is  possible  to  test  this  qualitative  inference  quanti- 
tatively and  see  if  the  falling  together  of  the  meteorites 
was  equal  to  the  task.  Knowing  the  mechanical  equiva- 
lent of  heat,  what  we  do  is  to  calculate  the  quantity  of 
motion  involved  and  then  evaluate  it  in  heat.  As  we 
are  unaware  of  the  exact  law  of  density  of  the  Earth,  and 
are  ignorant  of  how  much  was  radiated  away  in  the 
process,  the  problem  is  a  little  like  estimating  the  for- 
tune of  a  man  when  we  do  not  know  the  stocks  in  which 
he  has  invested,  and  ignore  how  much  he  has  spent  the 


1 52        THE   EVOLUTION  OF  WORLDS 

while.  We  only  know  what  he  would  have  been  worth 
had  he  followed  our  advice  in  the  matter  of  investments 
and  lived  as  frugally  as  we  recommended.  For  here, 
too,  we  are  obliged  to  make  certain  assumptions. 
Nevertheless  the  figure  obtained  in  the  case  of  the  plan- 
ets' stores  of  heat  is  so  enormous  as  to  leave  a  most 
ample  margin  for  dissipation.  Had  the  Earth  con- 
tracted from  a  fairly  generous  expansion  to  its  present 
state  under  the  probable  law  of  density  suggested  by 
Laplace  in  another  connection,  the  heat  developed 
would  have  been  enough  to  raise  the  whole  globe  to 
160,000°  F.  if  of  iron,  90,000°  F.  if  of  stone.  As  10,000° 
F.  would  have  sufficed  for  the  Earth  to  have  kept  up  its 
past,  to  say  nothing  of  its  present,  state,  we  are  justified 
of  our  deduction. 

Nor  is  the  Earth  the  only  body  in  the  system  which 
thus  argues  itself  evolved  by  the  falling  together  of  its 
present  constituents.  In  the  larger  planets  Jupiter  and 
Saturn  we  seem  to  see  the  heat,  far  as  we  are  away. 
For  the  cherry  hue  they  disclose  between  their  brighter 
belts  proves  to  come  from  greater  absorption  there  of 
the  green  and  blue  rays  of  the  spectrum,  indicating  a 
greater  depth  of  atmosphere  traversed.  Thus  these 
parts  lie  at  a  lower  level,  and  their  ruddy  hue  is  just  what 
they  should  show  were  they  still  glowing  with  a  dull  red 
heat. 

Heat  is  not  only  the  end  of  the  beginning,  it  is  the  be- 


FORMATION   OF   PLANETS  153 

ginning  of  the  end  as  well.  It  is  both  the  result  of  the 
evolving  of  definite  bodies  out  of  the  agglomeration  of 
matter-strewn  space,  and  the  cause  of  the  higher  evolu- 
tion of  those  globes  themselves.  For  the  acquisition  of 
heat  is  the  necessary  preface  to  all  that  follows.  Heat 
is  a  body's  evolutionary  capital  whose  wise  expenditure 
through  cooling  down  makes  all  further  advance  to 
higher  products  possible.  A  body  too  small  to  have 
acquired  it  must  remain  forever  lifeless,  as  dead  as  the 
meteorites  themselves  that  enter  our  air  as  mere  inert 
bits  of  stone  or  iron. 

Curiously  enough,  heat  both  must  have  been  and  then 
must  have  been  lost.  Like  the  loss  of  fortune  or  of 
friends  sometimes  in  the  ennobling  of  character,  it  is 
through  its  passing  away  that  its  effects  are  realized. 
For  in  cooling  down  from  a  once  heated  condition,  that 
train  of  events  occurs  which  we  most  commonly  particu- 
larize as  evolution.  So  far  in  our  survey  the  march  of 
advance  has  been  through  masses  of  matter,  a  molar 
evolution;  from  this  point  on  it  passes  into  its  minute 
constituents  and  becomes  a  molecular  one.  The  one 
is  the  necessary  prelude  to  the  other.  Up  to  this  great 
turning-point  in  the  history  of  each  member  of  a  solar 
system  we  have  been  busied  with  the  acquisition  of  heat, 
though  we  may  not  have  been  aware  of  it  the  while. 
All  the  motions  we  have  studied  tended  to  that  end. 
During  these  three  chapters,  I,  II,  V,  we  have  been 


1 54        THE   EVOLUTION   OF  WORLDS 

gradually  rising  in  our  point  of  view  until  we  stand  at 
the  temperature  pinnacle  of  the  whole  process.  In  the 
next  three  we  are  to  descend  upon  the  other  side.  The 
slope  we  have  come  up  was  of  necessity  barren;  the  one 
we  are  to  go  down  brings  us  to  verdure  and  the  haunts 
of  men.  Coming  from  the  causes  above,  we  reach  at 
each  step  effects  more  and  more  related  to  ourselves 
which  those  causes  will  help  us  to  explain. 


CHAPTER  VI 
A  PLANET'S  HISTORY 
Self-sustained  Stage 

UP  to  this  point  in  our  retrospective  survey  the  long 
course  of  evolution  has  taken  one  line,  that  of 
dynamical  separation  of  the  system's  parts  with  sub- 
sequent reunitement  of  them  according  to  the  laws  of 
celestial  mechanics.  Of  this  action  I  have  submitted 
the  reader  my  brief:  departing  in  it  from  common-law 
practice,  in  which  the  cause  of  action  is  short  and  the 
brief  long.  And  I  have,  I  trust,  guarded  against  his 
appealing  on  exceptions. 

From  this  point  on  we  have  two  kinds  of  develop- 
ment to  follow:  the  one  intrinsic,  the  chemical;  the 
other  incidental,  the  physical.  Not  that,  in  a  way,  the 
one  is  divorcible  from  the  other.  For  the  physical 
makes  possible  the  chemical  by  furnishing  it  the  con- 
ditions to  act.  But  in  another  sense,  and  that  which  is 
most  thrust  upon  our  notice,  the  two  are  independent. 
Thus  oceans  and  land,  hills  and  valleys,  clouds  and  blue 
sky,  as  we  know  them,  —  everything,  pretty  much, 
which  we  associate  with  a  world,  —  are  not  universal, 


156        THE   EVOLUTION   OF  WORLDS 

inevitable,  results  of  planetary  evolution,  but  resultant, 
individual,  characteristics  of  our  particular  abode. 
They  are  as  much  our  own  as  the  peculiar  arithmetic  of 
waiters  is  theirs,  or  as  used  to  be  the  sobriety  of  the 
country  doctor's  horse  —  his  and  no  other's.  Our 
whole  geologic  career  is  essentially  earthly.  Not  that 
its  fundamental  laws  are  not  of  universal  application, 
but  the  kaleidoscopic  patterns  they  produce  depend  on 
the  little  idiosyncrasies  of  the  constituents  and  the  mode 
in  which  these  fall  together.  Our  everyday  experiences 
we  should  find  quite  changed,  could  we  alight  on  Venus 
or  on  Mars. 

On  the  other  hand,  the  chemical  changes  which 
follow  a  body's  acquisition  of  heat,  setting  in  the  mo- 
ment that  heat  has  reached  its  acme  and  starts  to  de- 
cline, are  as  universal  as  the  universe  itself.  They  are 
conditioned,  it  is  true,  by  the  body's  size  and  by  the 
position  that  body  occupied  in  the  primal  nebula,  but 
they  depend  directly  upon  the  degree  of  heat  the  body 
had  attained.  The  larger  the  planet,  the  higher  the 
temperature  it  reached  and  the  fuller  its  possibilities. 
Even  the  planets  are  born  to  their  estate.  Thus  the 
little  meteorites  live  their  whole  waking  life  during  the 
few  seconds  they  spend  rushing  through  our  air.  For 
then  only  does  change  affect  their  otherwise  eternally 
inert  careers.  That  the  time  is  too  short  for  any  im- 
portant experience  is  evident  on  their  faces. 


A   PLANET'S   HISTORY  157 

Heat  is  most  intimately  associated  with  the  very  con- 
stitution of  matter.  It  is,  in  fact,  merely  the  motion 
of  its  ultimate  particles,  and  plays  an  essential  part  in 
their  chemical  relations.  Just  as  a  certain  discreet 
fervor  and  sufficient  exposure  for  attraction  to  take, 
make  for  matrimony,  so  with  the  little  molecules,  a 
suitable  degree  of  warmth  and  a  propitious  opportunity 
similarly  conduce  to  conjunction;  too  fiery  a  tempera- 
ment resulting  in  a  vagabondage  preventative  of  settled 
partnership  and  too  cold  a  one  in  permanent  celibacy. 
You  may  think  the  simile  a  touch  too  anthropomor- 
phic, but  it  is  a  most  sober  statement  of  fact.  Indeed, 
it  is  more  than  probable  that  in  some  dull  sense  they 
feel  the  impulse,  though  not  the  need  of  expressing  it  in 
verse.  That  metals  can  remember  their  past  states 
seems  to  have  been  demonstrated  by  Bose,  and  is  cer- 
tainly in  keeping  with  general  principles  as  we  know 
them  to-day.  For  memory  is  the  partial  retention  of 
past  changes,  rendering  those  changes  more  facile  of 
repetition. 

A  high  degree  of  heat,  then,  makes  chemical  union 
impossible,  because  the  great  speeds  at  which  the  mole- 
cules are  rushing  past  each  other  prevents  any  of  them 
being  caught.  Lack  of  speed  is  equally  deterrent. 
Nor  is  it  wholly  or  even  principally,  perhaps,  a  move- 
ment of  the  whole  which  is  here  concerned,  but  a  parti- 
tive throbbing  of  the  molecule  itself.  Certain  it  is  that 


158        THE   EVOLUTION  OF  WORLDS 

great  cold  is  as  prohibitive  of  chemic  combination  as 
great  heat.  Phosphorus,  which  evinces  such  avidity  for 
oxygen  at  ordinary  temperatures  as  to  have  got  its  name 
from  the  way  it  publishes  the  fact,  at  very  low  ones 
shows  a  coolness  for  its  affinity  amounting  to  absolute 
unconcern.  Thus  only  within  a  certain  range  of 
temperature  does  chemical  combination  occur.  To  re- 
main above  or  below  this  is  to  stay  forever  immortally 
dead.  To  get  hot  enough  in  the  first  place,  and  then 
subsequently  to  cool,  are  therefore  essential  processes 
to  a  body  which  is  to  know  evolutionary  advance. 

To  pen  the  history  of  the  solar  system  and  leave  out 
of  it  all  mention  of  its  most  transcendentally  wonderful 
result,  the  chemical  evolution  attendant  upon  cooling, 
would  be  to  play  "  Hamlet  "  with  Hamlet  left  out.  For 
the  thing  which  makes  the  second  half  of  the  great  cos- 
mic drama  so  inconceivably  grand  is  the  building  up 
of  the  infinitely  little  into  something  far  finer  than  the 
infinitely  great.  The  mechanical  action  that  first  tore 
a  sun  apart,  and  then  whirled  the  fragments  into  the 
beautifully  symmetric  system  we  behold  to-day,  is  of  a 
grandeur  which  is  at  least  conceivable;  the  molecular 
one  that,  beginning  where  the  other  left  off,  built  up 
first  the  diamond  and  then  humanity  is  one  that  passes 
our  power  to  imagine.  That  out  of  the  aggregation  of 
meteorites  should  come  man,  a  being  able  to  look  back 
over  his  own  genesis,  to  be  cognizant  of  it,  as  it  were, 


A   PLANET'S   HISTORY  159 

from  its  first  beginnings,  is  almost  to  prove  him  immanent 
in  it  from  the  start.  Fortunate  it  is  that  his  powers 
should  seem  more  limited  than  his  perceptions,  and  the 
more  so  as  he  goes  farther,  else  he  had  been  but  the 
embodiment  of  conceit. 

We  must  sketch,  therefore,  the  steps  in  this  marvel- 
lous synthesis ;  hastily,  for  I  have  already  spoken  of  it 
elsewhere  in  print  and  repetitions  dull  appreciation,  — 
in  the  appreciative,  —  though  we  have  the  best  of  prec- 
edents for  believing  that,  even  in  science,  to  be  dull 
and  iterative  insures  success;  the  dulness  passing  for 
wisdom  and  the  iteration  tiring  opposition  out. 

In  the  Sun  all  substances  are  in  their  elemental 
state.  Though  its  materials  are  the  same  as  the  Earth's, 
we  should  certainly  not  feel  at  home  there,  even  if  we 
waived  the  question  of  comfort,  for  we  should  recog- 
nize nothing  we  know.  We  talk  glibly  of  elements  as 
if  we  had  personal  acquaintance  with  them,  man's 
innate  snobbery  cropping  out.  For  to  the  chemist 
alone  are  they  observable  entities.  No  one  but  he  has 
ever  beheld  calcium  or  silicon,  or  magnesium,  or  man- 
ganese, and  most  of  us  would  certainly  not  know  these 
everyday  elements  if  we  met  them  on  the  street.  Of 
all  the  substances  composing  the  Earth's  crust,  or  the  air 
above,  or  the  water  beneath,  practically  the  only  ele- 
ments with  which  we  are  personally  familiar  are  iron, 
copper,  and  carbon,  and  these  only  in  minute  quantities 


160        THE   EVOLUTION   OF  WORLDS 

and  in  that  order  of  acquisition;  which  accounts  for 
the  stone,  iron,  and  bronze  ages  of  man,  ending  we 
may  add  with  the  graphite  or  lead-pencil  age  of  early 
education. 

Yet  that  elementary  substances  once  existed  here  we 
have  evidence.  We  find  such  in  volcanic  vents.  That 
the  Earth  was  once  as  hot  on  its  surface  as  it  now  is 
underneath,  we  know  from  the  condition  of  the  plutonic 
rocks  where  sedimentary  strata  have  not  covered  them 
up.  Volcanoes  and  geysers  are  our  only  avenues  now 
to  that  earlier  state  of  things.  From  these  pathways 
to  the  past,  and  only  from  them,  do  we  find  elementary 
substances  produced  to-day,  —  hydrogen,  sulphur,  chlo- 
rine, oxygen,  and  carbon.*  We  are  thus  made  aware 
that  once  the  Earth  was  simple,  too,  on  the  surface  as 
well  as  deeper  down.  A  side-light,  this,  to  what  we 
knew  must  have  been  the  case. 

From  its  primordial  state,  the  least  complex  com- 
pounds were  evolved  first.  As  the  heat  lessened,  higher 
and  higher  combinations  became  possible.  And  this  is 
why  the  more  complex  molecules  are  so  unstable,  the 
organic  ones  the  most.  Since  they  are  not  possible  at 
all  under  much  stir  of  their  atomic  constituents,  it 
shows  that  the  bond  between  them  must  be  feeble  — 
and,  therefore,  easily  broken  by  other  causes  besides 
heat.  To  the  instability  of  the  organic  molecule  is  due 
*  Geikie,  "  Geology,"  pages  85,  86,  and  131-136. 


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A   PLANET'S   HISTORY  161 

its  power;  and  to  cooling,  the  possibility  of  its  expres- 
sion. 

For  the  steps  in  the  chemical  process  from  Sun  to 
habitable  Earth  we  must  look  to  the  spectroscope;  not 
in  its  older  field,  the  blue  end  of  the  spectrum,  but  in 
that  which  is  unfolding  to  our  view  in  Dr.  Slipher's 
ingenious  hands,  the  extension  of  the  observable  part  of 
it  into  the  red.  For  at  that  end  lie  the  bands  due  to 
planetary  absorption.  Here  we  have  already  secured 
surprising  results  as  to  the  atmospheres  of  the  various 
planets.  We  have  not  only  found  positive  evidence  of 
water-vapor  in  the  atmosphere  of  Mars,  but  we  have 
detected  strange  envelopes  in  the  major  planets  which 
show  a  constitution  different  from  that  of  the  Sun  on  the 
one  hand,  and  of  the  Earth  on  the  other.  That  size  and 
position  are  for  much  in  these  peculiarities,  I  have  al- 
ready shown  you ;  but  something,  too,  is  to  be  laid  at  the 
door  of  age.  The  major  planets  are  not  so  advanced  in 
their  planetary  history  as  is  our  Earth;  and  Dr.  Sli- 
pher's spectrograms  of  them  disclose  what  is  now  going 
on  in  that  prefatory,  childish  stage. 

These  spectrograms  are  full  of  possibilities,  and  it  is 
not  too  much  to  say  that  chemistry  may  yet  be  greatly 
indebted  to  the  stars.  Compounds,  the  strange  un- 
known substances  there  revealed  by  their  spectral  lines, 
may  be  cryptic  as  yet  to  us.  Some  of  the  elements  miss- 
ing in  MendeleefFs  table  may  be  there,  too.  Helium 


1 62        THE   EVOLUTION   OF  WORLDS 

was  first  found  in  the  Sun;  coronium  still  awaits  de- 
tection elsewhere.  So  with  these  spectral  lines  of  the 
outer  planets.  It  looks  as  if  chemistry  had  been  a 
thought  too  previous  in  making  free  for  others  with 
what  should  have  been  their  names,  Zenon  and  Ura- 
nium. For  we  may  yet  have  to  speak  of  Dion  and 
Varunium. 

From  the  chemical  aspect  of  evolution  we  pass  to  its 
physical  side ;  from  the  indirectly  to  the  directly  visible 
results.  Here  again,  to  learn  what  happened  after  the 
sunlike  stage,  we  must  turn  to  the  major  planets. 
For  the  cooling  which  induced  both  physical  and  chemi- 
cal change  has  there  progressed  less  far,  inasmuch  as  a 
large  globe  takes  longer  to  cool  than  a  small  one.  To 
the  largest  planets,  then,  we  should  look  for  types  of  the 
early  planetary  stages  to-day. 

Almost  as  soon  as  the  telescope  was  directed  to 
Jupiter,  among  the  details  it  disclosed  were  the  Jovian 
belts  (in  the  year  1630),  dark  streaks  ruling  the  planet's 
disk  parallel  to  its  equator.  They  are  of  the  first  ob- 
jects advertised  as  visible  in  small  glasses  to-day,  vying 
with  the  craters  in  the  Moon  as  purchasable  wonders  of 
the  sky.  As  the  belts  were  better  and  better  seen,  fea- 
tures came  out  in  them  which  proved  more  and  more 
interesting.  Cassini,  in  1692,  noticed  that  the  mark- 
ings travelled  round  Jupiter  and  those  nearest  his 
equator  the  quickest.  Sir  William  Herschel  thought 


A  PLANETS   HISTORY  163 

them  due  to  Jovian  trade-winds,  the  planet's  swift 
rotation  making  up  for  deficiency  of  sun ;  why,  does  not 
appear. 

Modern  study  of  the  planet  shows  that  the  bright  lon- 
gitudinal layers  between  the  dark  belts  are  unquestion- 
ably belts  of  cloud.  Their  behavior  indicates  this,  and 
their  intrinsic  brightness  bears  it  out.  For  they  are  of 
almost  exactly  that  albedo.  Whether  they  are  the  kind 
of  cloud  with  which  we  are  familiar,  clouds  of  water- 
vapor,  we  are  not  yet  sure.  But  whatever  their  con- 
stitution, their  conduct  is  quite  other  than  is  exhibited 
by  our  own. 

In  the  first  place,  they  are  of  singular  permanence  for 
clouds.  The  fleeting  forms  we  know  as  such  assume 
in  the  Jovian  air  a  stability  worthy  of  Jove  himself.  In 
their  general  outlines,  they  remain  the  same  for  years 
at  a  time.  "Constant  as  cloud"  would  be  the  proper 
poetic  simile  there.  But  while  remaining  true  to  them- 
selves, they  prove  to  be  in  slow,  unequal  shift  with  one 
another.  Thus  Jupiter's  official  day  differs  according 
to  the  watch  of  the  particular  belt  that  times  it.  Spots 
in  different  latitudes  drift  round  lazily  in  appearance, 
swiftly  in  fact,  those  near  the  equator  as  a  rule  the  fast- 
est. Nor  is  there  any  hard  and  fast  latitudinal  law; 
it  is  a  go-as-they-please  race  in  which  one  belt  passes  its 
neighbor  at  a  rate  sometimes  of  four  hundred  miles  an 
hour.  The  mean  day  is  Qh  55™  long. 


1 64        THE   EVOLUTION   OF  WORLDS 


A  side-light  is  cast  upon  the  Jovian  state  of  things  by 
the  "  great  red  spot,"  which  has  been  more  or  less  visible 

for  thirty  years,  and  which 
takes  five  minutes  longer 
than  the  equatorial  band  to 
travel  round.  Its  tint  be- 
spoke interest  in  what  might 
be  its  atmospheric  horizon. 
Yet  it  betrayed  no  sign  of 
JUPITER  AND  ITS  "GREAT  RED  being  either  depressed  or 

SPOT  "   A     DRAWING      BY     DR.  1          1  '    1  J  1 

LOWELL,   APRIL   I2,    7*  o--5«",      CXalted   WItH    regard     tO    the 

I9°7'  rest    of    the    surface.     "  In 

1891,"  as  Miss  Clerke  puts  it,  "  an  opportunity  was 
offered  of  determining  its  altitude  relative  to  a  small 
dark  spot  on  the  same  par- 
allel, by  which,  after  months 
of  pursuit,  it  was  finally 
overtaken.  An  occultation 
appeared  to  be  the  only  alter- 
native from  a  transit;  yet 
neither  occurred.  The 
dark  spot  chose  a  third.  It 
coasted  round  the  obstacle 
in  its  way,  and  got  damaged 
beyond  recognition  in  the  process."  It  thus  astutely 
refused  to  testify. 

Now,  this  exclusiveness  on  the  part  of  the    "great 


JUPITER    AND   ITS    "  GREAT    RED 

SPOT "  A    DRAWING    BY   DR. 

LOWELL,  APRIL  12,  7h  28m-42m, 
1907. 


A   PLANETS   HISTORY  165 

red  spot "  really  offers  us  an  insight  to  its  character. 
Clearly  it  was  no  void,  but  occupied  space  with  more 
than  ordinary  persistency.  As  it  was  neither  above 
nor  below  the  dark  spot  and  shattered  that  spot  on 


SUN   SPOTS AFTER   BOND. 

approach,  which  its  former  surroundings  had  not  done, 
its  force  must  have  been  due  to  motion.  This  can 
be  explained  by  its  being  formed  of  a  vast  uprush  of 
heated  vapor  from  the  interior.  In  short,  it  was  a  sort 
of  baby  elephant  of  a  volcano,  or  geyser,  occurring  as 
befits  its  youth  in  fluid,  not  solid,  conditions,  but  fairly 
permanent,  nevertheless  —  a  bit  of  kindergarten  Jovian 
geology.  This  estimate  of  it  is  concurred  in  by  Dr. 


1 66        THE   EVOLUTION   OF   WORLDS 


Slipher's  spectrogram  of  the  dark  and  light    belts  re- 
spectively.    For  in  the  spectrum  of  the  dark  one  we  see 

the  distinctive 
Jovian  bands  in- 
tensified as  if  the 
light  had  trav- 
ersed a  greater 
depth  of  Jovian 
air.  Its  color,  a 
cherry  red,  abets 
the  conclusion 
-that  in  such 
places  we  look 
down  into  the 
fiery,  chaotic 
turmoil  so  inces- 
santly going  on. 
It  is  of  inter- 
est to  note  that 
we  have  proto- 
types of  this  sort 
of  extraterrestrial  cyclone  in  the  Sun.  His  spots  are 
probably  local  upsettings  of  atmospheric  equilibrium, 
using  the  word  atmospheric  in  the  widest  possible 
sense.  Just  as  our  storms  are  the  mildest  examples 
of  the  like  expostulation  at  the  impossibility  of  keep- 
ing up  a  too  long  continued  decorum.  Only  that  with 


PHOTOGRAPH  OF  A  SUN  SPOT  —  AFTER  THE  LATE 
M.  JANSSEN. 


A   PLANET'S   HISTORY  167 

us  the  Earth  is  not  so  much  to  blame  as  the  Sun; 
while  both  Jupiter  and  the  Sun  are  themselves  respon- 
sible for  their  condition. 

Thus  we  have,  in  the  very  depth  of  their  negation, 
warrant  from  the  dark  belts  of  Jupiter  that  the  bright 
ones  are  cloud.  But  also  that  they  are  not  clouds 
ordered  as  ours.  The  Jovian  clouds  pay  no  sort  of 
regard  to  the  Sun.  In  orbital  matters  Jupiter  obeys 
the  ruler  of  the  system;  but  he  suffers  no  interference 
from  him  in  his  domestic  affairs.  His  cloud-belts  be- 
have as  if  the  Sun  did  not  exist.  Day  and  night  cause 
no  difference  in  them ;  nor  does  the  Jovian  year.  They 
come  when  they  will;  last  for  months,  years,  decades; 
and  disappear  in  like  manner.  They  are  sui  Jovis, 
caused  by  vertical  currents  from  the  heated  core  and 
strung  out  in  longitudinal  procession  by  Jupiter's  spin. 
They  are  self-raised,  not  sun-raised,  condensations  of 
what  is  vaporized  below.  Jove  is  indeed  the  cloud- 
compeller  his  name  implies. 

Yet  Jupiter  emits  no  light,  unless  the  cherry  red  of 
his  darker  belts  be  considered  its  last  lingering  glow. 
He  is  thus  on  the  road  from  Sun  to  world,  and  his  pres- 
ent appearance  informs  us  that  this  incubation  takes 
place  under  cloud. 

The  like  is  true  of  Saturn,  in  fainter  replica,  even  to 
the  cherry  hue.  In  one  way  Saturn  visibly  asserts  his 
independence  beyond  that  possible  by  Jupiter.  For 


1 68        THE   EVOLUTION   OF   WORLDS 

Jupiter's  equator  lies  almost  in  the  plane  of  his  orbit,  and 
on  a  hasty  view  the  Sun  might  be  credited  with  the  or- 
dering of  the  belts,  as  was  indeed  long  the  case.  But 
Saturn's  inclination  to  his  orbital  plane  is  27°;  yet  his 
belts  fit  his  figure  as  neatly  as  his  rings,  and  never  get 
displaced,  no  matter  how  his  body  be  turned. 

Uranus  and  Neptune  are  in  the  same  self-centred 
attitude  at  present  as  the  faint  traces  of  belts  on  their 
disk,  otherwise  of  the  same  albedo  as  cloud,  lead  us  to 
conclude.  Yet  both  their  densities  and  their  situation 
give  us  to  believe  them  further  advanced  than  the  giant 
planets,  and  still  they  lie  wrapped  in  cloud. 

These  planets,  then,  are  quite  unbeholden  to  the  Sun 
for  all  their  present  internal  economies.  What  goes  on 
under  that  veil  of  clouds  with  which  they  discreetly  hide 
their  doings  from  the  too  curious  astronomic  eye  —  we 
can  only  conjecture.  But  we  discern  enough  to  know 
that  it  is  no  placid  uneventfulness.  That  it  will  con- 
tinue, too,  we  are  assured.  For  whether  these  clouds 
are  largely  water-vapor  now,  or  not,  to  watery  ones 
they  must  come  as  the  last  of  all  the  wrappers  they 
will  eventually  put  off. 

The  major  planets  are  the  only  ones  at  the  present 
moment  in  this  self-centred  and  self-sustained  stage. 
Their  great  size  has  kept  them  young.  In  the  smaller 
terrestrial  planets  we  could  not  expect  to  witness  any 
such  condition  to-day.  If  they  experienced  an  ebullient 


A   PLANET'S   HISTORY 


169 


youth,  they  have  long  since  outgrown  it.  Only  by 
rummaging  their  past  could  we  find  evidence  on  the 
point,  and  this,  distance  both  in  time  and  space  bars 


THE  VOLCANO  COLIMA,  MEXICO,  MARCH  24,  1903  —  JOSE 
MARIA  ARREOLA,  PER  FREDERICK  STARR. 

us  from  doing.  There  is  but  one  body  into  whose 
foretime  career  we  could  hope  to  peer  with  the  slightest 
prospect  of  success  —  our  own  Earth. 

Whether  our  Earth  was  ever  hot  enough  at  the  surface 
to  vaporize  those  substances  which  now  form  the  Jovian 


1 70        THE   EVOLUTION  OF  WORLDS 


or  Saturnian  clouds,  we  do  not  know;  but  that  it  was 
once  hot  enough  to  vaporize  water  we  are  perfectly 
certain.  And  this  from  proof  both  of  what  did  exist 


JUKES  BUTTE,  A  DENUDED  LACCOLITH,  AS  SEEN  FROM  THE 

NORTHWEST GILBERT. 

and  of  what  did  not.  That  the  surface  temperature 
was  at  one  time  in  the  thousands  of  degrees  Fahrenheit, 
the  Plutonic  magma  underlying  all  the  sedimentary 
rocks  of  the  Earth  amply  shows.  Reversely,  the  ab- 
sence of  any  effect  of  water  until  we  reach  these  sedi- 
mentary deposits,  testifies  that  during  all  the  earlier 
stages  of  the  Earth's  career  water  as  such  was  absent, 
and  as  water  subsequently  appeared,  it  is  clear  that  the 

conditions  did  not  at  first 
allow  it  to  form.  We  are 
sure,  therefore,  that  there 
was  a  time  when  water  ex- 
IDEAL  SECTION  OF  A  LACCOLITH—  isted  only  as  steam,  and  very 

GILBERT.  ' 

possibly  a  period  still  an- 
terior to  that  when  it  did  not  exist  at  all,  its  constitu- 
ent hydrogen  and  oxygen  not  having  yet  combined. 
There  was  certainly  an  era,  then,  in  the  morning  of  the 


A   PLANET'S   HISTORY  171 

ages,  when  the  Earth  wore  her  cloud-wrapper  much  as 
Jupiter  his  now. 

That  the  seas  were  not  once  and  yet  are  to-day,  affords 
proof  positive  that  at  some  intermediate  period  they  be- 
gan to  be.  A  very  long  intermediate  one  it  must  have 
been,  too,  —  all  the  time  it  took  the  Earth  to  cool  from 
about  2000°  C.  to  1 00°  C.  Not  till  after  the  temperature 
had  fallen  to  the  latter  figure  in  the  outer  regions  of  the 
atmosphere  could  clouds  form,  and  not  till  it  had  done 
so  at  the  solid  surface  could  the  steam  be  deposited  as 
water.  Reasoning  thus  presents  us  with  a  picture  of 
our  Earth  as  a  vast  seething  caldron  from  which  steam 
condensing  into  cloud  was  precipitated  upon  a  heated 
layer  of  rock,  to  rise  in  clouds  of  steam  again.  The 
solid  surface  had  by  this  time  formed,  thickening  slowly 
and  more  or  less  irregularly,  and  into  its  larger  dimples 
the  water  settled  as  it  grew,  deepening  them  into  the 
great  ocean  basins  of  to-day.  We  see  the  process  with 
as  much  certainty  and  considerably  more  comfort  than 
if,  in  the  French  sense,  we  had  assisted  at  it.  Presence 
of  mind  now  thus  amply  makes  up  for  absence  of  body 
then. 

Passing  on  evolutionary  we  reach  more  and  more 
tolerable  conditions  and  solid  ground  in  fact,  as  well  as 
theory.  Thus  the  crust  hardened  and  cooled,  while  the 
oceans  still  remained  uncomfortably  hot.  For  water 
requires  much  more  heat  to  warm  it  to  a  given  tempera- 


172 


THE   EVOLUTION   OF   WORLDS 


ture  than  rock,  about  four  and  a  half  times  as  much. 
It  has  therefore  by  so  much  the  more  to  lose,  and  is  pro- 
portionally long  in  the  losing.  These  hot  seas  must 
have  produced  a  small  universe  of  cloud,  and  as  the 
conditions  were  the  same  all  over  the  Earth,  we  can  see 
easily  with  the  mind's  eye  that  we  could  not  have  seen 
at  all  with  the  bodily  one,  had  we  occupied  the  land  in 
those  very  early  days.  To  be  quite  shut  out  from  curi- 
ous sight  without,  was  hardly  made  up  for  by  not  being 
able  to  see  more  than  dimly  within.  Any  one  who  has 
stood  on  the  edge  of  a  not-extinct  crater  when  the  wind 
was  blowing  his  way,  will  have  as  good  a  realization  of 
the  then  state  of  things  as  he  probably  cares  for. 

Now  this  astronomic  drawing  of  the  then  Earth, 
which  by  its  lack  of  detail  allows  of  no  doubt  whatever, 
permits  us  to  offer  help  in  the  elucidation  of  some  of 
their  phenomena  to  our  geologic  colleagues.  We  are 
the  more  emboldened  to  do  so  in  that  they  have  them- 
selves appealed  to  astronomy  for  diagnosis,  and  accepted 
nostrums  devised  by  themselves.  It  is  always  better  in 
such  cases  to  call  in  a  regular  practitioner.  Not  that  he 
is  necessarily  more  astute,  but  that  he  knows  what  will 
not  work.  It  was  in  the  matter  of  the  paleologic  climate 
that  they  were  led  to  consult  astronomy.  The  singular 
thing  about  paleologic  times  was  the  combination  of 
much  warmth  with  little  light;  and  the  not  less  singular 
fact  that  these  conditions  were  roughly  uniform  over 


A   PLANET'S   HISTORY  173 

the  whole  Earth.  From  this  universality  it  was  clear, 
as  De  Lapparent,  their  chief  spokesman,  puts  it,  that 
nothing  local  could  explain  the  fact.  It  was  something 
which  demanded  a  cause  common  to  the  globe. 

It  thus  fell  properly  within  the  province  of  astronomy. 
For  if  we  are  to  draw  any  line  between  the  spheres  of 
influence  of  the  two  sciences,  it  would  seem  to  lie  where 
totality  ends  and  provincialism  begins.  I  use  this  not 
as  a  pejorative,  but  simply  to  part  local  color  from  one 
universal  drab.  In  the  Earth's  general  attributes,  — its 
size,  shape,  and  weight, — we  must  have  recourse  to 
astronomy  to  learn  the  facts.  Not  less  so  for  those  prin- 
cipal causes  which  have  shaped  its  general  career;  we 
surrender  it  only  at  the  point  where  everyday  interest 
begins,  when  those  causes  that  led  it  through  its  uninvit- 
ing youth  give  way  to  effects  which  in  the  least  concern 
humanity  at  large. 

Between  the  mere  aggregation  of  matter  into  planet- 
ary bodies,  of  which  nebular  hypotheses  treat,  and  the 
specific  transformation  of  plants  and  animals  upon 
their  surfaces  with  which  organic  evolution  is  concerned, 
lies  a  long  history  of  development,  which,  beginning  at 
the  time  the  body  starts  to  cool,  continues  till  it  be- 
come, for  one  cause  or  another,  again  an  inert  mass. 
In  this  period  is  contained  its  career  as  a  world. 
Planetology  I  have  ventured  to  call  the  brand  of  as- 
tronomy which  deals  with  this  evolution  of  worlds.  It 


174        THE   EVOLUTION   OF   WORLDS 

treats  of  what  is  general  and  cosmic  in  that  evolution, 
as  geology  treats  of  what  is  terrestrial  and  specific  in 
the  history  of  one  member  of  the  class,  our  own  Earth. 
The  two  do  not  interfere,  as  the  one  faces  questions  in 
time  and  space  to  which  the  other  remains  perforce  a 
stranger.  If  the  picture  by  the  one  be  fuller  of  detail, 
the  canvas  of  the  other  permits  of  the  wider  perspec- 
tive. Certain  events  in  the  history  of  our  Earth  can 
only  be  explained  by  astronomy,  as  geologists  have 
long  since  recognized.  It  is  these  that  fall  into  our 
present  province. 

Geologists,  however,  have  applied  astronomy  accord- 
ing to  their  own  ideas.  Either  they  called  in  aurists,  so 
to  speak,  when  what  they  needed  was  an  oculist,  or  they 
went  to  books  for  their  drugs,  which  they  then  ad- 
ministered themselves  —  a  somewhat  dangerous  prac- 
tice. Thus  they  began  by  displacing  the  Earth's  axis 
in  hope  of  effecting  a  result;  not  realizing  that  this 
would  only  shift  the  trouble,  not  cure  it;  in  fact,  make 
it  rather  worse.  They  next  tried  what  De  Lapparent, 
one  of  the  most  brilliant  geologists  of  the  age,  calls  "  a 
variation  in  the  eccentricity  of  the  ecliptic  *  joined  to 
precession  of  the  equinoxes/'  — a  startling  condition  un- 
known to  astronomy  which  does  not  deal  in  eccentric 
planes,  whatever  such  geometric  anomalies  may  be,  but 
by  which  its  coiner  evidently  means  a  change  in  the 

*  "  Abrege  de  Geologic,"  De  Lapparent. 


A  PLANET'S   HISTORY  175 

eccentricity  of  the  orbit,  as  the  context  shows.  Its 
effect  on  the  Earth,  as  he  wisely  points  out,  would  be  to 
reduce  its  extremities  to  extremes.  To  get  out  of  his 
quandary  he  then  embraced  a  brilliant  suggestion  of  a 
brother  geologist,  M.  Blandet.  M.  Blandet  conceived 
the  idea,  and  brought  it  forth  unaided,  that  all  that  was 
necessary  was  a  sun  big  enough  to  look  down  on  both 
poles  of  the  Earth  at  once.  To  get  this  he  travelled  back 
to  the  time  when,  in  Laplace's  cosmogony,  the  Sun  filled 
the  whole  orbit  of  Mercury.  This  conception,  which, 
De  Lapparent  remarks,  "  might,  at  the  time  of  its  ap- 
parition, have  disconcerted  spirits  accustomed  to  con- 
sider our  system  as  stable,"  -  an  apparition  which  we 
may  add  would  certainly  continue  to  disconcert  them,  — 
he  says  seems  to  him  quite  in  harmony  with  that  sys- 
tem's genesis.  That  it  labors  under  two  physical  im- 
possibilities, one  on  the  score  of  the  Sun,  the  other  on 
that  of  the  Earth,  and  that  in  this  case  two  negatives 
do  not  make  an  affirmative,  need  not  be  repeated  here, 
as  the  reader  will  find  it  set  forth  at  length  elsewhere,* 
together  with  what  I  conceive  to  be  the  only  explana- 
tion of  paleothermal  times  which  will  work  astronomi- 
cally —  presently  to  be  mentioned.  But  before  I  do 
so,  it  is  pertinent  to  record  two  things  that  have  come  to 
my  notice  since.  One  is  that  in  rereading  Faye's 
"  Origine  du  Monde,"  I  came  upon  a  passage  in  which 
*  "  Mars  as  the  Abode  of  Life,"  Macmillan,  1908. 


176        THE   EVOLUTION   OF   WORLDS 

it  appears  that  M.  Blandet  had  actually  consulted  Faye 
about  his  hypothesis,  and  that  Faye  had  shown  him  its 
impossibility  on  much  the  same  grounds  as  those  above 
referred  to;  which,  however,  did  not  deter  M.  Blandet 
from  giving  it  to  the  world  nor  De  Lapparent  from  god- 
fathering the  conception. 

Faye,  meanwhile,  developed  his  theory  of  the  origin 
of  the  world,  and  by  it  explained  the  greater  heat  and 
lesser  light  of  paleologic  times  compared  with  our  own, 
thus :  The  Earth  evolved  before  the  Sun.  In  paleologic 
times  the  Sun  was  still  of  great  extent,  —  an  ungathered- 
up  residue  of  nebula  that  had  not  yet  fallen  together 
enough  to  concentrate,  not  a  contracting  mass  from 
which  the  planets  had  been  detached,  —  and  was  in 
consequence  but  feebly  luminous  and  of  little  heating 
effect;  so  that  there  were  no  seasons  on  Earth  and  no 
climatic  zones.  The  Earth  itself  supplied  the  heat 
felt  uniformly  over  its  whole  surface. 

This  differs  from  my  conception,  as  the  reader  will 
see  presently,  in  one  vital  point  —  as  to  why  the  Earth 
was  not  heated  by  the  Sun.  In  the  first  place  F  aye's 
sun  has  no  raison  d'etre;  and  in  the  second  no  visible 
means  of  existence.  If  its  matter  were  not  already 
within  the  orbit  of  the  Earth  at  the  time,  there  seems  no 
reason  why  it  should  ever  get  there ;  and  if  there,  why 
it  should  have  been  so  loath  to  condense.  We  cannot 
admit,  I  think,  any  such  juvenility  in  the  Sun  at  the  time 


TREE  FERN. 


A   PLANET'S   HISTORY  177 

the  Earth  was  already  so  far  advanced  as  geology  shows 
it  to  have  been  in  paleologic  times.  For  the  Earth  had 
already  cooled  below  the  boiling-point  of  water. 

To  understand  the  problem  from  the  Earth's  point  of 
view,  let  us  review  the  facts  with  which  geology  presents 
us.  The  flora  of  paleologic  times,  as  we  see  both  at 
their  advent  in  the  Devonian  and  from  their  superb 
development  in  the  Carboniferous  era,  consisted  wholly 
of  forms  whose  descendants  now  seek  the  shade.*  Tree 
ferns,  sigillaria,  equisetae,  and  other  gloom-seeking 
plants  composed  it.  That  some  tree-fern  survivals  to- 
day can  bear  the  light  does  not  invalidate  the  racial  ten- 
dency. We  have  plenty  of  instances  in  nature  of  such 
adaptability  to  changed  conditions.  In  fact,  the  dying 
out  and  deterioration  of  most  of  the  order  shows 
that  the  conditions  have  changed.  And  these  plants, 
grown  to  the  dimensions  of  trees,  inhabited  equally  the 
tropic,  the  temperate,  and  the  frigid  zones  as  we  know 
them  now.  Lastly,  no  annual  rings  of  growth  are  to  be 
found  on  them.-)-  In  other  words,  they  grew  right  on, 
day  in,  day  out.  The  climate,  then,  was  as  continuous 
as  it  was  widespread. 

On  the  other  hand,  astronomy  and  geology  both  as- 
sert that  the  seas  were  warm.f  From  this  it  follows 

*  De  Lapparent,  Dana,  Geikie,  passim. 

f  De  Lapparent. 

J  De  Lapparent,  Dana,  Geikie,  passim. 

N 


178        THE   EVOLUTION   OF   WORLDS 

that  a  vastly  greater  evaporation  must  have  gone  on 
then  than  now,  and  that  a  welkin  of  cloud  must  thus 
inevitably  have  been  formed. 

Now  put  the  two  facts  together,  and  you  have  the  solu- 
tion. The  climate  was  warm  and  equable  over  the  whole 
globe  because  a  thick  cloud  envelope  shut  off  the  Sun's 
heat,  the  heat  being  wholly  supplied  from  the  steamy 
seas.  At  the  same  time,  by  the  same  means  the  light 
was  necessarily  so  tempered  as  to  produce  exactly  that 
half-light  the  ferns  so  dearly  love.  One  and  the  same 
cause  thus  answers  the  double  riddle  of  greater  warmth 
and  less  light  in  those  old  days  than  is  now  the  case. 

And  here  comes  in  the  second  find  I  spoke  of  above, 
in  the  person  of  some  old  trilobites  who  stepped  in  un- 
expectedly in  corroboration.  It  has  long  been  known 
—  though  its  full  significance  seems  to  have  escaped 
notice  —  that  in  1872  M.  Barrande  made  the  discovery 
that  many  species  of  trilobites  of  the  Cambrian  and 
lower  Silurian,  the  two  lowest,  and  therefore  the  oldest, 
strata  of  paleozoic  times,  and  distant  relative  of  our 
horseshoe  crabs,  were  blind.  What  is  yet  more  signifi- 
cant, the  most  antediluvian  were  the  least  provided  with 
eyes.  Thus  in  the  primordial  strata,  one-fourth  of 
the  whole  number  of  species  were  eyeless,  in  the  next 
above  one-fifth,  and  in  the  latest  of  all  one  two-hundredth 
only.*  Furthermore,  they  testify  to  the  difficulty  of 

*  Suess,  "  The  Face  of  the  Earth,"  p.  213. 


A   PLANET'S   HISTORY  179 

seeing,  in  two  distinct  ways,  some  by  having  no  eyes 
and  some  colossal  ones,  strenuous  individuals  increasing 
their  equipment  and  the  lazy  letting  it  lapse.  It  seems 
more  than  questionable  to  attribute  this  blindness  to  a 
deep-sea  habitat,  as  Suess  does  in  describing  them, 
for  they  lived  in  what  geologists  agree  were  shallow 
seas  on  the  site  of  Bohemia  to-day.  Besides,  trilobites 
never  had  abyssal  proclivities;  for  they  are  found  pre- 
served in  littoral  deposits,  not  in  deep-sea  silt.  Muddy 
water  may  have  had  some  hand  in  this,  but  muddy 
water  itself  testifies  to  great  commotion  above  and  tor- 
rential rains.  So  the  light  in  those  seas  was  not  what  it 
became  later,  or  would  be  now.  Thus  these  trilobites 
were  antelucan  members  of  their  brotherhood,  and  this 
accuses  a  lack  of  light  in  those  earlier  eras  even  greater 
than  in  Carboniferous  times,  which  is  just  where  it 
ought  to  be  found  if  the  theory  is  true. 

I  trust  this  conception  may  prove  acceptable  to  geol- 
ogists, for  it  seems  imperative  from  the  astronomic 
side  that  something  of  the  sort  must  have  occurred. 
And  it  is  just  as  well,  if  not  better,  to  view  it  thus  in  the 
light  of  the  dawn  of  geologic  history  as  to  remain  in  the 
dark  about  it  altogether.  Nescience  is  not  science  - 
whether  hyphenized  or  apart;  for  the  whole  object  of 
science  is  to  synthesize  and  explain.  Its  body  of  learn- 
ing is  but  the  letter,  coordination  the  spirit,  of  its  law. 
Nevertheless,  the  unpardonable  impropriety  of  a  new 


i8o        THE   EVOLUTION   OF  WORLDS 

idea,  I  am  aware,  is  as  reprehensible  as  the  atrocious 
crime  of  being  a  young  man.  Yet  the  world  could  not 
get  on  without  both.  Time  is  a  sure  reformer  and 
will  render  the  most  hardened  case  of  youth  senile  in 
the  end.  So  even  a  new  idea  may  grow  respectable  at 
last.  And  it  is  really  as  well  to  make  its  acquaint- 
ance while  it  still  has  vigor  in  it  as  to  wait  till  it  is 
old  and  may  be  embraced  with  impunity.  Boasted 
conservatism  is  troglodytic,  and  usually  proves  a 
self-conferred  euphuism  for  dull.  For  conservatism 
proceeds  from  slowness  of  apprehension.  It  may  be 
necessary  for  certain  minds  to  be  in  the  rear  of  the 
procession,  but  it  is  of  doubtful  glory  to  find  distinction 
in  the  fact. 

Thus  the  youth  of  a  world,  like  the  babyhood  of  an 
individual,  is  passed  screened  from  immediate  contact 
from  without.  That  this  is  the  only  way  that  life  can 
originate  on  a  planet  we  cannot  say,  but  that  it  is  away 
in  which  it  does  occur,  our  own  Earth  attests,  arid 
that,  moreover,  it  is  the  way  with  all  planets  of  sufficient 
size,  the  present  aspect  of  the  major  planets  shows.  It 
may  well  be  that  with  celestial  bodies  as  with  earthly 
species,  some  swaddle  their  young,  others  cast  them 
forth  to  take  their  chance,  and  that  those  that  most  pro- 
tect them  rear  the  higher  progeny  in  the  end.  What 
glories  in  evolution  thus  await  the  giant  planets  when 
they  shall  have  sufficiently  cooled  down,  we  can  only 


A   PLANET'S   HISTORY  181 

dimly  imagine.  But  we  can  foresee  enough  to  realize 
that  we  are  not  the  sum  of  our  solar  system's  possibili- 
ties, and  by  studying  the  skies  read  there  a  future  more 
wonderful  than  anything  we  know. 


CHAPTER  VII 
A  PLANET'S  HISTORY 

Sun-sustained  Stage 

TWO  stages  have  characterized  the  surface  history 
of  the  Earth, — stages  which  may  be  likened  to  the 
career  of  the  chick  within  and  without  the  egg.  In  the 
first  of  them  the  Earth  lay  screened  from  outside  in- 
fluence under  a  thick  shell  of  cloud,  indifferently  ex- 
clusive of  the  cold  of  space  or  of  the  heating  beams  of  the 
Sun.  Motherless,  the  warmth  of  its  own  body  brooded 
over  it,  keeping  its  heat  from  dissipating  too  speedily 
into  space,  and  so  fostering  the  life  that  was  quickening 
upon  its  surface. 

The  second  stage  began  when  the  egg-shell  broke  and 
the  chick  lay  exposed  to  the  universe  about  it,  to  get  its 
living  no  longer  from  its  little  world  within,  but  from 
the  greater  one  without.  One  and  the  same  event  ended 
the  old  life  to  make  possible  the  new.  So  soon  as  the 
cloud  envelope  was  pierced,  both  the  Earth's  own  heat 
escaped  and  the  Sun's  rays  were  permitted  to  come  in. 

It  is  not  surprising  that  under  such  changed  condi- 
tions development  itself  should  have  changed,  too.  In 

182 


A   PLANET'S   HISTORY  183 

fact,  the  transformation  was  marked.  That  its  epochal 
character  has  failed  to  impress  itself  generally  on 
geologists,  is  perhaps  because  they  look  too  closely, 
missing  the  march  of  events  in  the  events  themselves, 


EARTH  AS  SEEN  FROM  ABOVE  —  PHOTOGRAPHED  BY  DR.  LOWELL  AT  AN 
ALTITUDE  OF  5500  FEET. 

and  because,  too,  of  the  gradual  nature  of  its  proces- 
sional change.  We  can  recall  only  De  Lapparent  as 
having  particularly  signalled  it;  although  not  only  in 
its  cause,  but  for  its  effects,  it  should  have  delimited 
two  great  geologic  divisions  of  time. 

Astronomy  and  geology  are  each  but  part  of  one  uni- 
versal history.  The  tale  each  has  to  tell  must  prove  in 
keeping  with  that  of  the  other.  If  they  seem  at  vari- 
ance, it  behooves  us  very  carefully  to  scan  their  respec- 
tive stories  to  find  the  flaw  where  the  apparent  incon- 
gruity slipped  in.  Each,  too,  fittingly  supplements  the 
other,  and  especially  must  geology  look  to  astronomy 


1 84        THE   EVOLUTION  OF  WORLDS 

for  its  initial  data,  since  astronomy  deals  with  the  be- 
ginning of  our  own  Earth. 

That  study  of  our  Earth  in  its  entirety  falls  properly 
within  the  province  of  astronomy,  is  not  only  deducible 
from  its  relationship  to  the  other  planets,  but  demon- 
strable from  the  cosmic  causes  that  have  been  at  work 
upon  it,  and  the  inadequacy  of  anything  but  cosmic  laws 
to  explain  them.  The  ablest  geologists  to-day  are  be- 
coming aware  of  this,  —  we  have  one  of  them  at  the 
head  of  the  geology  department  of  the  Institute,  — 
while  from  the  curious  astronomy  at  second  hand  which 
gets  printed  in  geologic  text-books,  by  eminent  men 
at  that,  dating  from  some  time  before  the  flood, — of 
modern  ideas, — it  seems  high  time  that  the  connection 
should  be  made  clear. 

For,  after  all,  our  Earth  too  is  a  heavenly  body,  in 
spite  of  man's  doing  his  best  to  make  it  the  reverse.  It 
has  some  right  to  astronomic  regard,  even  if  it  is  our  own 
mother.  At  the  same  time  it  is  quite  puerile  to  consider 
the  universe  as  bounded  by  our  terrestrial  backyard. 
If  man  took  himself  a  thought  less  importantly,  he  might 
perceive  the  humor  of  so  circumscribed  a  view.  Like 
children  we  play  at  being  alone  in  the  universe,  and  then 
go  them  one  better  by  believing  it  too. 

I  shall,  of  course,  not  touch  on  any  matters  purely 
geologic,  for  fear  of  committing  the  very  excesses  I 
deplore;  mentioning  only  such  points  as  astronomy  has 


A   PLANET'S   HISTORY  185 

information  on,  and  which,  by  the  sidelights  it  throws, 
may  help  to  illuminate  the  subject. 

Thus  it  certainly  is  interesting  and  may  to  many  be  a 
new  point  of  view,  that  the  changes  introduced  when 
paleologic  times  passed  into  neologic  ones  were  in  their 
fundamental  aspects  essentially  astronomic;  which 
shows  how  truly  astronomic  causes  are  woven  into  the 
whole  fabric  of  the  Earth.  For  it  was  then  only, 
terrestrially  speaking,  that  the  year  began.  The  or- 
bital period  had  existed,  of  course,  from  the  time  the 
Earth  first  made  the  circuit  of  the  Sun.  But  the  year 
was  more  a  succes  cTestime  on  the  Sun's  part  than  one  of 
popular  appreciation.  As  the  Sun  could  not  be  seen 
and  worked  no  striking  effects  upon  the  Earth,  the 
annual  round  had  no  recognizable  parts,  and  one  revo- 
lution lapsed  into  the  next  without  demarcation.  Only 
with  the  clearing  of  the  sky  did  the  seasons  come  in : 
to  register  time  by  stamping  its  record  on  the  trees. 
Before  that,  summer  and  winter,  spring  and  autumn, 
were  unknown. 

Climate,  too,  made  then  its  first  appearance;  climate, 
named  after  the  sunward  obliquity  of  the  Earth,  and 
seeming  at  times  to  live  down  to  that  characterization. 
Weather  there  had  been  before;  pejoratively  speaking, 
nothing  but  weather.  For  the  downpours  in  paleo- 
logic times  must  have  been  exceeded  in  numbers  only 
by  their  force.  One  dull  perpetual  round  of  rain  was 


1 86        THE   EVOLUTION   OF  WORLDS 

the  programme  for  the  day,  with  absolutely  no  hope  of 
a  happy  clearance  to-morrow.  It  was  the  golden  age 
only  for  weather  prophets  whose  prognostications  could 
hardly  go  wrong.  With  climate,  however,  it  was  a  very 
different  matter.  With  polyp  corals  building  reefs 
almost  to  the  pole  (81°  50'),*  as  far  north  nearly  as  man 
has  yet  by  his  utmost  efforts  succeeded  in  getting,  while 

their   fellows  were   busy   at   the   like   industry   in   the 

« 

tropics,  it  is  clear  that  latitude  was  laughed  at  and  cli- 
mate even  lacked  a  name. 

Another  astronomic  feature,  then  for  the  first  time 
disclosed,  was  the  full  significance  of  the  day  and  the 
revelation  of  its  cause.  While  the  Earth  brooded  under 
perpetual  cloud,  there  could  have  been  but  imperfect 
recognition  of  day  and  night.  Or  perhaps  we  may  put 
it  better  by  saying  that  the  standard  of  both  was  greatly 
depressed,  dull  days  alternating  with  nights  black  as 
pitch.  But  the  moment  the  Sun  was  let  in,  all  this 
changed,  though  not  in  a  twinkling.  The  change  came 
on  most  gradually.  We  can  see  in  our  mind's  eye  the 
first  openings  in  the  great  welkin  permitting  the  Earth 
its  initial  peeps  of  the  world  beyond,  and  how  quickly 
and  tantalously  they  shut  in  again  like  a  mid-storm 
morning  which  dreams  of  clearing  only  to  find  how 
drowsy  it  still  is.  But  eventually  the  clouds  parted 
afresh  and  farther,  and  the  Earth  began  to  open  its 
eyes  to  the  universe  without. 

*  Dana,  "  Geology." 


A   PLANET'S   HISTORY  187 

The  cause  of  the  clearing,  of  course,  was  the  falling 
temperature  of  the  seas.  Evaporation  went  on  much 
less  fast  as  the  heat  of  the  water  lessened.  The  whole 
round  of  aquatic  travel  from  ocean  to  air,  and  back  to 
ocean  again,  proceeded  at  an  everslackening  pace.  And 
here,  if  it  so  please  geologists,  may  be  found  a  reconcil- 
ing of  their  demands  for  time  to  the  relative  pittance 
astronomy  has  been  willing  to  dole  them  out,  a  paltry 
50  or  100  millions  of  years,  which  like  all  framers  of 
budgets  they  have  declared  utterly  insufficient.  For 
in  early  times  the  forces  at  work  were  greater,  and 
by  magnifying  the  means  you  quicken  the  process  and 
contract  the  Earth's  earlier  eras  to  reasonable  limits. 

Upon  these  various  astronomic  novelties,  the  Earth 
on  thus  awakening  looked  for  the  first  time.  Such  re- 
gard altered  for  good  its  own  internal  relations.  The 
wider  outlook  made  impossible  the  life  of  the  narrower 
that  preceded  it.  A  totally  changed  set  of  animals  and 
plants  arose,  to  whom  the  cosmos  bore  a  different  as- 
pect. The  Earth  ceased  to  be  the  self-centred  spot  it 
seemed  before.  As  long  ago  as  this  had  the  idea  that 
our  globe  was  the  centre  of  the  universe  been  cosmi- 
cally  exploded.  The  Earth  knew  it  if  man  did  not. 

Its  denizens  responded.  The  organisms  that  already 
inhabited  it  proceeded  to  change  their  character  and 
crawl  out  upon  the  land.  For  in  Devonian  times  the 
Earth  was  the  home  of  fishes.  The  land  was  not  con- 


1 88        THE   EVOLUTION   OF  WORLDS 

sidered  a  fit  abode  by  anything  but  insects,  and  not  over- 
good  by  them.  But  it  looked  different  when  the  Sun 
shone.  Some  maritime  dwellers  felt  tempted  to  ex- 
plore, and  proceeded  in  the  shape  of  amphibians  to  spy 


TRACKS  OF  SAUROPUS  PRIM^VUS  (X  J).  I.  LEA.  —  DANA,  "MANUAL  OF 
GEOLOGY." 

out  the  land.  They  have  left  very  readable  accounts 
of  their  travels  in  foot-notes  by  the  way.  As  one  should 
always  inspect  the  original  documents,  I  will  reproduce 
the  foot-notes  of  one  early  explorer.  It  is  one  of  the  few 
copies  we  have,  as  the  type  is  worn  out.  But  it  tells  a 
pretty  full  story  as  it  stands.  The  ripple-marks  show 
that  a  sea  beach  it  was  which  the  discoverer  trod  in  his 
bold  journey  of  a  few  feet  from  home  and  friends,  and  the 
pits  in  the  sandstone  that  it  was  raining  at  the  time  of  his 
excursion.  No  Columbus  or  Hakluyt  could  have  left 
a  record  more  precise  or  more  eminently  trustworthy. 
The  pilgrims  found  it  so  good  that  their  eventual 
collaterals,  the  great  reptiles,  actually  took  possession 
of  the  land  and  held  it  for  many  centuries  by  right  of 
eminent  domain.  Yet  throughout  the  time  of  these 


A   PLANET'S   HISTORY  189 

bold  adventurers,  their  skies  were  only  clearing,  as  the 
pitting  of  the  sandstone  eloquently  states. 

It  was  not  till  the  chalk  cliffs  of  Dover  were  being 
laid  down  that  we  have  evidence  that  seasons  had  fully 
developed,  in  the  shape  of  the  first  deciduous  trees.* 
Cryptogams,  cycads,  and,  finally,  conifers  had  in  turn 
represented  the  highest  attainments  of  vegetation,  and 
the  last  of  these  had  already  recognized  the  seasons  by 
a  sort  of  half-hearted  hibernation  or  annual  moulting; 
deeming  it  wise  not  to  be  off  with  the  old  leaves  before 
they  were  on  with  the  new.  But  finally  the  most  ad- 
vanced among  them  decided  unreservedly  to  accept  the 
winter  and  go  to  sleep  till  spring.  The  larches  and 
ginkgo  trees  are  descendants  of  the  leaders  of  this 
coniferous  progressive  party. 

At  the  same  time  color  came  in.  We  are  not  ac- 
customed to  realize  that  nature  drew  the  Earth  in  grays 
and  greens,  and  touched  it  up  with  color  afterward. 
Only  the  tempered  tints  of  the  rocks  and  the  leaden 
blue  of  the  sea,  subdued  by  the  disheartening  welkin 
overhead  to  a  dull  drab,  enlivened  their  abode  for  the 
oldest  inhabitants.  But  with  Tertiary  times  entered 
the  brilliantly  petalled  flowers.  Beginning  with  yellow, 
these  rose  through  a  chromatic  scale  of  beauty  from 
white  through  red  to  blue.f  They  decked  themselves 

*  Dana,  Geikie,  De  Lapparent. 
f  Cf.  Grant  Allen. 


1 9o        THE   EVOLUTION   OF  WORLDS 

thus  gaudily  because  the  Sun  was  there  to  see  by,  as 
well  as  eyes  to  see.  For  without  the  Sun  those  uncon- 
scious horticulturists,  the  insects,  could  not  have  exer- 
cised their  pictorial  profession. 

To  the  entering  of  the  Sun  upon  the  scene  this  won- 
drous revolution  was  due;  and  once  entered,  it  became 
the  dominant  factor  in  the  Earth's  organic  life.  We 
are  in  the  habit  of  apostrophizing  the  Sun  as  the  source 
of  all  terrestrial  existence.  It  is  true  enough  to-day,  and 
has  been  so  since  man  entered  on  the  scene.  But  it  was 
not  always  thus.  There  was  a  time  when  the  Sun 
played  no  part  in  the  world's  affairs. 

As  its  heat  is  now  all-important,  it  becomes  an  in- 
teresting matter  to  determine  the  laws  governing  its 
amount.  That  summer  is  hotter  than  winter  we  all 
know  from  experience,  pleasurable  or  painful  as  the 
case  may  be.  This  is  due  to  the  fact  that  the  Sun  is 
above  the  horizon  for  a  greater  number  of  hours  in 
summer  and  passes  more  directly  overhead.  But  not  so 
many  people  are  aware  that  on  midsummer  day,  so  far 
as  the  Sun  is  concerned,  the  north  pole  should  be  the 
hottest  place  on  earth.  That  Arctic  explorers,  who 
have  got  within  speaking  acquaintance  of  it,  assure  us 
it  is  not  so,  shows  that  something  besides  the  direct  rays 
of  the  Sun  is  involved.  Indeed,  we  learn  as  much  from 
the  extensively  advertised  thermometers  of  winter  re- 
sorts which,  judiciously  placed,  beguile  the  stranger  to 


A   PLANET'S   HISTORY  191 

sojourn  where  it  is  just  too  cold  for  comfort.  The 
factor  in  question  is  the  blanketing  character  of  our  air. 
Now  a  blanket  may  keep  heat  out  as  well  as  keep  it  in. 
Our  air  acts  in  both  capacities.  It  is  by  no  means 
simply  a  storer  of  heat,  as  many  people  seem  to  suppose; 
it  is  a  heat-stopper  as  well.  What  it  really  is  is  a  tempo- 
rizer, a  buffer  to  ease  the  shocks  of  sudden  change  like 
those  comfortable,  phlegmatic  souls  who  reduce  all  emo- 
tion to  a  level.  For  the  heating  power  of  the  Sun, 
even  at  the  Earth's  distance  away,  is  much  greater  than 
appears.  Knowledge  of  this  we  owe  most  to  Langley, 
and  then  to  Very,  who  continued  his  results  to  yet  a 
finer  determination,  the  best  we  have  to-day.  In  con- 
sequence we  have  learnt  that  the  amount  of  heat  we 
should  receive  from  the  Sun,  could  we  get  above  our  air, 
-the  solar  constant,  as  it  is  called, — would  be  over 
three  times  what  it  is  on  the  average  in  our  latitude  at 
the  surface,  and  is  rising  still,  so  to  speak.  For  as  man 
has  gone  higher  he  has  found  his  inferences  rising  too, 
and  the  limit  would  seem  to  be  not  yet.  We  see  then 
that  the  air  to  which  we  thought  ourselves  so  much  in- 
debted, actually  begins  its  kindly  offices  by  shutting 
off  two-thirds  of  what  was  coming  to  us.  As  it  plays, 
however,  something  of  the  same  trick  to  what  tries  to 
escape,  we  are  really  somewhat  beholden  to  it  after  all. 
But  not  so  much  as  has  been  thought.  We  used  to  be 
told  that  the  Moon's  temperature  even  at  midday  hardly 


1 92        THE   EVOLUTION   OF  WORLDS 

rose  above  freezing,  but  Very  has  found  it  about  350° 
F.,  which  even  the  most  chilly  of  souls  might  find  warm. 
By  the  late  afternoon,  however,  he  would  need  his  over- 
coat, and  no  end  of  blankets  subsequently,  for  during 
the  long  lunar  night  of  fourteen  days  the  temperature 
must  fall  appallingly  low,  to  —  300°  F.  or  less. 

As  the  determination  of  temperature  is  a  vital  one, 
not  only  to  any  organic  existence,  but  even  to  inorganic 
conditions  upon  a  planet,  it  behooves  us  to  look  care- 
fully into  the  question  of  the  effective  heat  received  from 
the  Sun.  Until  recently  the  only  criterion  in  the  case 
was  assumed  to  be  distance  from  the  illuminating  source, 
about  as  efficient  a  mode  of  computation  as  estimating 
a  Russian  army  by  its  official  roll.  For  as  we  saw  in 
our  own  case,  not  all  that  ought  to  ever  gets  to  the  front, 
to  say  nothing  of  what  is  lost  there.  Yet  on  this  worse 
than  guesswork  astronomic  text-books  still  assert  as  a 
fact  that  the  temperature  of  other  bodies  —  the  Moon 
and  Mars,  for  example  —  must  be  excessively  low. 

Let  us  now  examine  into  this  most  interesting  prob- 
lem. It  is  intricate,  of  course,  but  I  think  you  will  find 
it  more  comprehensible  than  you  imagine.  Indeed,  I 
shall  be  to  blame  if  you  do  not.  For  if  one  knows  his 
subject,  he  can  always  explain  it,  in  untechnical  lan- 
guage, technical  terms  being  merely  a  sort  of  short- 
hand for  the  profession.  The  physical  processes  in- 
volved can  be  made  clear  without  difficulty,  although 


A   PLANETS   HISTORY 

their  quantitative  evaluation  is  less  forthrightly  demon- 
strable. Let  me,  then,  give  you  an  epitome  of  my  in- 
vestigation of  the  subject. 

Consider  a  ray  of  light  falling  on  a  surface  from  the 


ATMOSPHERE 


ADVENTURES  OF  A  HEAT  RAY. 


Sun.  A  part  of  it  is  reflected;  that  is,  is  instantly 
thrown  off  again.  By  this  part  the  body  shines  and 
makes  its  show  in  the  world,  but  gets  no  good  itself. 
Another  part  is  absorbed;  this  alone  goes  to  heat 
the  body.  Now  if  the  visible  rays  were  all  that  ema- 
nated from  the  Sun,  it  would  be  strictly  true,  and  a 


194        THE   EVOLUTION   OF  WORLDS 

pretty  paradox  for  believers  in  the  efficacy  of  distance, 
that  what  heated  the  planet  was  precisely  what  seemed 
not  to  do  so.  Unfortunately  there  are  also  invisible 
rays,  and  these,  too,  are  in  part  reflected  and  in  part 
absorbed,  and  their  ratio  is  different  from  that  of  the 
visible  ones.  To  appreciate  them,  Langley  invented 
the  bolometer,  in  which  heat  falling  on  a  strip  of  metal 
produces  a  current  of  electricity  registered  by  a  gal- 
vanometer. By  thus  recording  the  heat  received  at 
different  parts  of  the  spectrum  and  at  different  heights 
in  our  atmosphere,  he  was  able  to  find  how  much 
the  air  cut  off.  Very  has  since  determined  this  still 
more  accurately.  By  thus  determining  the  depletion 
in  the  invisible  part  of  the  spectrum  joined  to  what  as- 
tronomy tells  us  of  the  loss  in  the  visible  part,  we  have 
a  value  for  the  whole  amount.  By  knowing,  then,  the 
immediate  brightness  of  a  planet  and  approximately 
the  amount  of  atmosphere  it  owns,  we  are  enabled  to 
judge  how  much  heat  it  actually  receives.  This  proves 
to  be,  in  the  case  of  Mars,  more  than  twice  as  much 
as  distance  alone  would  lead  us  to  infer. 

The  second  question  is  how  much  of  this  it  retains. 
The  temperature  of  a  body  at  any  moment  is  the  balance 
struck  between  what  it  receives  and  what  it  radiates. 
If  it  gets  rid  of  a  great  deal  of  its  income,  it  will  clearly 
be  less  hot  than  if  it  is  miserly  retentive.  To  find 
how  much  it  radiates  we  may  take  the  difference  in 


A   PLANET'S   HISTORY  195 

temperature  between  sunset  and  sunrise,  since  during 
this  interval  the  Earth  receives  no  heat  from  the  Sun. 
In  the  same  way  the  efficacy  of  different  atmospheric 
blankets  may  be  judged.  Thus  the  Earth  parts  with 
nine  centigrade  degrees'  worth  of  its  store  on  clear 
nights,  and  only  four  degrees'  worth  on  cloudy  ones, 
before  morning.  This  is  at  sea-level.  By  going  up  a 
high  mountain  we  get  another  set  of  depletions,  and 
from  this  a  relative  scale  for  different  atmospheric 
blankets.  This  is  the  principle,  and  we  only  have  to 
fill  out  the  skeleton  of  theory  with  appropriate  num- 
bers to  find  how  warm  the  body  is. 

In  doing  so,  we  light  on  some  interesting  facts.  Thus 
clouds  reflect  72  per  cent  of  the  visible  rays,  letting 
through  only  28  per  cent  of  them.  We  feel  chilly  when 
a  cloud  passes  over  the  Sun.  On  the  other  hand,  slate 
reflects  only  18  per  cent  of  the  visible  rays,  absorbing  all 
the  rest.  This  is  why  slate  gets  so  much  hotter  in  the 
Sun  than  chalk,  and  why  men  wear  white  in  the  tropics. 
White,  indeed,  is  the  best  color  to  clothe  one's  self  in  the 
year  around,  except  for  the  cold  effect  it  has  on  the  im- 
agination, for  it  keeps  one's  own  heat  in  as  well  as  keep- 
ing the  Sun's  out.  The  modest,  self-obliterating,  white 
winter  habit  of  the  polar  hares  not  only  enables  them  to 
keep  still  and  escape  notice,  but  keeps  them  warm  while 
they  wait. 

Astronomically,  the  effect  is  equally  striking.     Mars, 


1 96        THE   EVOLUTION   OF   WORLDS 

for  example,  owing  to  being  cloudless  and  of  a  duller 
hue,  turns  out  to  have  a  computed  mean  temperature 
nearly  equal  to  the  Earth's, —  a  theoretic  deduction  which 
the  aspect  of  the  planet  most  obligingly  corroborates. 
It  thus  enjoys  a  comparatively  genial  old  age. 

For  what  is  specially  instructive  in  planetary  economy 
is  that,  on  the  whole,  clear  skies  add  more  by  what  they 
let  in  than  they  subtract  by  what  they  let  out.  If  the 
Earth  had  no  clouds  at  all,  its  mean  temperature  would 
be  higher  than  it  is  to-day.  Thus  as  a  planet  ages  a 
beneficent  compensation  is  brought  about,  the  Sun's 
heat  increasing  as  its  own  gives  out.  Not  that  the 
foreign  importation,  however  slight  the  duty  levied  on  it 
by  the  air,  ever  fully  makes  up  for  the  loss  of  the  domes- 
tic article,  but  it  tempers  the  refrigeration  which  inevi- 
tably occurs. 

The  subject  of  refrigeration  leads  us  to  one  of  the  most 
puzzling  and  vexed  problems  of  geology:  how  to  ac- 
count for  the  great  Ice  Age  of  which  the  manifest  sign 
manuals  both  in  Europe  and  in  America  have  so  in- 
trigued man  since  he  began  to  read  the  riddle  of  the 
rocks.  Upon  this,  also,  planetology  throws  some  light. 

If  I  needed  an  apology  to  the  geologists  for  seeming 
again  to  trespass  on  their  particular  domain,  I  might 
refer  to  the  astrocomico  expositions  put  forward  to 
account  for  the  great  Ice  Age. 

We  can  all  remember  Croll's  "Climate  and  Time/' 


A   PLANET'S   HISTORY  197 

a  book  which  can  hardly  be  overpraised  for  its  title  and 
which  had  things  worth  reading  inside,  too.  It  had  in 
consequence  no  inconsiderable  vogue  at  one  time.  It 
undertook  to  account  for  glacial  epochs  on  astronomic 
principles.  It  called  in  such  grand  cosmic  conditions 
and  dealt  in  such  imposing  periods  of  time  that  it  fired 
fancy  and  almost  compelled  capitulation  by  the  mere 
marshalling  of  its  figurative  array.  Secular  change  in 
the  eccentricity  of  the  Earth's  orbit,  combined  with 
progression  in  the  orbital  place  of  the  winter's  solstice, 
was  supposed  to  have  induced  physical  changes  of  cli- 
mate which  accentuated  the  snowfall  in  the  northern 
hemisphere  and  so  caused  extensive  and  permanent 
glaciation  there.  In  other  words,  long,  cold  winters 
followed  by  short,  hot  summers  in  one  hemisphere  were 
credited  with  accumulating  a  perpetual  snow  sheet, 
such  as  short,  warm  winters  and  long,  cold  summers 
could  not  effect. 

Now  it  so  happens  that  these  astronomic  conditions 
affecting  the  Earth  several  thousand  years  ago,  are  in 
process  of  action  on  one  of  our  nearest  planetary 
neighbors  at  the  present  time.  The  orbit  of  Mars 
is  such  that  its  present  eccentricity  is  greater  than 
what  the  Earth  ever  can  have  had,  and  the  winter 
solstice  of  the  planet's  southern  hemisphere  falls 
within  23°  of  its  aphelion  point.  We  have  then  the 
conditions  for  glaciation  if  these  are  the  astronomic 


198        THE   EVOLUTION   OF   WORLDS 

ones  supposed,  and  we  should  expect  a  southern  polar 
cap,  larger  at  its  maximum  and  still  more  so,  rela- 
tively, at  its  minimum,  than  in  the  opposite  hemisphere. 
Let  us  now  look  at  the  facts,  for  we  have  now  a  knowl- 


MARS. 

NORTH  POLAR  CAP.  SOUTH  POLAR  CAP. 

At  maximum        full  extent  of  white  At  maximum        white 

At  minimum         inner  circle  At  minimum         nothing 

edge  of  the  Martian  polar  caps  exceeding  in  some  re- 
spects what  we  know  of  our  own.  The  accompanying 
diagrams  exhibit  the  state  of  things  at  a  glance,  the 
maximum  and  minimum  of  each  cap  being  represented 
in  a  single  picture  and  the  two  being  placed  side  by  side. 
It  will  be  observed  that  the  southern  cap  outdoes  its 
antipodal  counterpart  at  its  maximum,  showing  that  the 
longer,  colder  winter  has  its  effect  in  snow  or  hoar-frost 
deposition.  But,  on  the  other  hand,  instead  of  excelling 


A   PLANET'S   HISTORY  199 

it  at  its  minimum,  which  it  should  do  to  produce  perma- 
nent glaciation,  it  so  far  falls  short  of  its  fellow  that 
during  the  last  opposition  at  which  it  could  be  well  ob- 
served, it  disappeared  entirely.  The  short,  hot  summer, 
then,  far  exceeded  in  melting  capacity  that  of  the  longer 
but  colder  one. 

Let  us  now  suppose  the  precipitation  to  be  increased, 
the  winters  and  summers  remaining  both  in  length  and 
temperature  what  they  were  before.  The  amount  of 
snow  which  a  summer  of  given  length  and  warmth  can 
dispose  of  is,  roughly  speaking,  a  definite  quantity.  For 
it  depends  to  a  great  extent  only  on  its  amount  of  heat. 
The  summer  precipitation  may  be  taken  as  offsetting 
itself  in  the  two  hemispheres  alike.  If,  then,  the  snow- 
fall in  the  winter  be  for  any  reason  increased  daily  in 
both,  a  timewill  come  when  the  deposition  due  the  longer 
winter  of  the  one  will  exceed  what  its  summer  can  melt 
relatively  to  the  other,  and  a  permanent  glaciation  re- 
sult in  the  hemisphere  so  circumstanced.  Increased 
precipitation,  then,  not  eccentricity  of  orbit,  is  the  real 
cause  of  an  Ice  Age.  And  this  astronomic  deduction  we 
owe  not  to  theoretic  conclusions,  for  which  we  lack  the 
necessary  quantitative  data,  but  wholly  to  study  of  our 
neighbor  in  space.  Had  any  one  informed  our  geologic 
colleagues  that  they  must  look  to  the  sky  for  definite 
information  about  the  cause  of  an  Ice  Age,  they  would 
probably  have  been  surprised. 


200        THE   EVOLUTION   OF  WORLDS 

With  this  Martian  information,  received  some  years 
ago,  it  is  pleasing  now  to  see  that  Earthly  knowledge  is 
gradually  catching  up.  For  that  increased  precipitation 
could  account  for  it,  the  evidence  of  pluvial  eras  in  the 


GLACIAL  MAP  OF  EURASIA  —  AFTER  JAMES  GEIKIE. 

equatorial  regions,  contemporaneous  with  glacial  periods, 
indicates.  But  another  and  probably  the  chief  factor 
involved  was  not  a  generally  increased  precipitation, 
potent  as  that  would  be,  but  an  increased  snow  deposit 
due  to  temporary  elevation  of  the  ground. 

For  it  now  appears  that  there  was  no  glacial  epoch. 
Our  early  ideas  inculcated  by  text-books  at  school  re- 
ceived a  rude  shock  when  it  appeared  that  the  glacial 
epoch  was  not,  as  we  had  been  led  to  believe,  a  polar 


A   PLANET'S   HISTORY 


201 


phenomenon  at  all,  but  a  local  affair  which  on  the  face 
of  it  had  nothing  to  do  with  the  pole.  For  investiga- 
tion has  disclosed  that  instead  of  emanating  from  the 
pole  southward,  it 
proceeded  from 
certain  centres,  de- 
scending thence  in 
all  directions, 
north  as  much  as 
south.  Thus 
there  was  a  centre 
in  Norway  in  65° 
N.  lat.  and  an- 
other in  Scotland 
in  56°  N.  In 
North  America 
there  were  three  — 
the  Labradorian 
in  latitude  54°  N., 
the  Kerwatin  to 
the  northwest  of 
Hudson's  Bay  in 
latitude  62°  N.,  and  the  Cordilleran  along  the  Pacific 
coast  in  latitude  58°  N.  On  the  other  hand,  northern 
Siberia,  the  coldest  region  in  the  world,  was  not  glaci- 
ated. That  the  ice  flowed  off  these  centres  proves 
them  to  have  been  higher  than  the  sides.  But  we  have 


MAP  SHOWING  THE  GLACIATED  AREA  OF  NORTH 
AMERICA  —  THE  ARROWS  INDICATING  THE  DI- 
RECTION OF  ICE  MOVEMENT CHAMBERLIN 

AND  SALISBURY. 


202        THE   EVOLUTION   OF   WORLDS 

further  evidence  of  their  then  great  height  from  the 
fact  that  dead  littoral  shells  have  been  dredged  from 
1333  fathoms  in  the  North  Atlantic,  and  the  prolonga- 
tion under  water  of  the  fiords  of  Norway  and  of  land 
valleys  in  North  America  witness  to  the  same  sub- 
sidence since. 

But  evidence  refuses  to  stop  here.  The  Alps  were 
then  more  glaciated  than  they  are  now.  So  was  Kili- 
manjaro and  Ruwenzori  on  the  equator;  and  finally  at 
the  same  time  more  ice  and  snow  existed  round  about 
the  south  pole  than  is  the  case  to-day.  Now  this  is 
really  going  too  far  even  for  the  most  ardent  believers 
in  the  force  of  eccentricity.  For  if  the  astronomic 
causes  postulated  were  true,  they  must  have  produced 
just  the  opposite  action  at  the  antipodes,  to  say  nothing 
of  the  crux  of  being  equally  effective  at  the  equator. 
The  theory  lies  down  like  the  ass  between  two  burdens. 
Whichever  load  it  chooses  to  saddle,  it  must  perforce 
abandon  the  other. 

So  it  turns  out  that  the  Ice  Age  was  not  an  Ice  Age  at 
all  but  an  untoward  elevation  of  certain  spots,  and  is  to 
be  relegated  to  the  same  limbo  of  exaggeration  of  a 
local  incident  into  a  world-wide  cataclysm  as  the  deluge. 
That  some  geologists  will  still  cling  to  their  former  belief 
I  doubt  not;  for  as  the  philosophic  old  lady  remarked: 
"There  always  have  been  two  factions  on  every  sub- 
ject. Just  as  there  are  the  suffragists  and  anti-sufFra- 


A   PLANET'S   HISTORY  203 

gists  now,  so  there  were  slaveholders  and  the  anti- 
slavery  people  in  my  time;  and  even  in  the  days  of  the 
deluge,  there  were  the  diluvians  who  were  in  favor  of  a 
flood  and  the  antediluvians  who  were  opposed  to  it." 
A  tale  which  has  a  peculiarly  scientific  moral,  as  in 
science  anti  and  ante  seem  often  interchangeable  terms. 

When  I  began  the  course  of  lectures  that  resulted  in 
this  volume,  I 'labored  under  the  apprehension  that  an 
account  of  cosmic  physics  might  prove  dull.  It  soon 
threatened  to  prove  too  startling.  I  therefore  hasten  to 
reassure  the  timid  by  saying  that  we  are  outgrowing  ice 
ages  and  probably  deluges.  Elevations  of  the  Earth's 
crust  are  likely  to  be  less  and  less  pronounced  in  the 
future,  and  meanwhile  such  as  exist  are  being  slowly 
worn  down.  Secondly,  the  Sun  is  sure  to  continue 
of  much  the  same  efficiency  for  many  aeons  to  come. 
And  lastly,  the  essential  ingredient  of  both  prodigies, 
water,  is  daily  becoming  more  scarce.  To  this  latter 
point  we  now  turn,  and  perhaps  when  it  is  explained  to 
him  the  reader  may  think  that  he  has  been  rescued  from 
one  fate  only  to  fall  into  the  hands  of  another. 

Geology  is  necessarily  limited  in  its  scope  to  what 
has  happened ;  planetology  is  not  so  circumscribed  in 
its  domain.  It  may  indulge  in  prognostication  of  the 
future,  and  find  countenance  for  its  conclusions  in  the 
physiognomy  of  other  worlds.  Thus  one  of  the  things 
which  it  foresees  is  the  relative  drying  up  of  our  abode. 


204        THE   EVOLUTION   OF   WORLDS 

To  those  whose  studies  have  never  led  them  off  this 
earth,  the  fact  that  the  oceans  are  slowly  evaporating  into 
space  may  seem  as  incredible  as  would,  to  one  marooned 
on  a  desert  island,  the  march  of  mankind  in  the  mean- 
time. We  live  on  an  island  in  space,  but  can  see  some- 
thing of  the  islands  about  us,  and  our  conception  of  what 
is  coming  to  our  limited  habitat  can  be  judged  most 
surely  by  what  we  note  has  happened  to  others  more 
advanced  than  ourselves.  Just  as  we  look  at  Jupiter  to 
perceive  some  likeness  of  what  we  once  were,  the  real 
image  of  which  has  travelled  by  this  time  far  into  the 
depths  of  space  beyond  possibility  of  recall,  so  must  we 
look  to  the  Moon  or  Mars  if  we  desire  to  see  some  faint 
adumbration  of  the  pass  to  which  we  are  likely  to  come. 
For  from  their  lack  of  size  they  should  have  preceded 
us  on  the  road  we  are  bound  to  travel.  Now,  both  these 
worlds  to-day  are  water-lacking,  in  whole  or  part;  the 
Moon  practically  absolutely  so,  Mars  so  far  as  any 
oceans  or  seas  are  concerned.  We  should  do  wisely 
then  to  take  note.  But  we  have  more  definite  informa- 
tion than  simply  their  present  presentments.  For  both 
bear  upon  their  faces  marks  of  having  held  seas  once 
upon  a  time.  They  were  once,  then,  more  as  we  are 
now.  We  cannot  of  course  be  sure,  as  we  are  unable  to 
get  near  enough  to  scan  their  surfaces  for  signs  of  erosive 
action.  But  so  far  as  we  can  make  out,  past  seas  best 
explain  their  appearance. 


A   PLANET'S   HISTORY 


205 


So  sealike,  indeed,  was  their  look  that  the  first  astron- 
omers to  note  them  took  them  unhesitatingly  for  water 
expanses.  Thus  the  moment  the  telescope  brought  the 
Moon  near  enough  for  map  making  of  it  we  find  the 
dark  patches  at  once 
designated  as  seas. 
The  Sea  of  Serenity, 
the  Sea  of  Showers, 
the  Bay  of  Rainbows, 
speak  still  of  what 
once  was  supposed  to 
be  the  nature  of  the 
dark,  smooth,  lunar 
surfaces  they  name. 
Suggestively,  indeed, 
in  an  opera  glass  do 
they  seem  to  lap  the  land.  The  Lake  of  Dreams  fore- 
shadowed what  was  eventually  to  be  thought  of  them. 
With  increasing  optical  approach  the  substance  evapo- 
rated, but  the  form  remained.  It  was  speedily  evident 
that  there  was  no  water  there;  yet  the  semblance  of  its 
repository  still  lurked  in  those  shadows  and  suggests  it- 
self to  one  scanning  their  surfaces  to-day.  If  they  be  not 
old  sea  bottoms,  they  singularly  mimic  the  reality  in  their 
smooth,  sloping  floors  and  their  long,  curving  lines  of 
beach.  Their  strange  uniformity  shows  that  something 
protected  them  from  volcanic  fury  while  the  rest  of  the 


THE  MOON  —  PHOTOGRAPHED  AT  THE 
LOWELL  OBSERVATORY. 


2o6        THE   EVOLUTION   OF  WORLDS 

lunar  face  was  being  corrugated.  This  preservative 
points  to  some  superincumbent  pressure  which  can  have 
been  no  other  than  water.  Lava-flows  on  such  a  scale 
seem  inadmissible.  What  these  surfaces  show  and  what 
they  do  not  show  alike  hint  them  sea  bottoms  once 
upon  a  time.  In  the  strange  chalk-like  hue  of  the  lunar 
landscape  they  look  like  plaster  of  Paris  death-masks  of 
the  former  seas. 

A  like  history  fell  to  the  lot  of  the  surface  features  of 
Mars.  There  too,  as  soon  as  the  telescope  revealed 
them  and  their  permanency  of  place,  the  dark  patches 
upon  the  planet's  face  were  forthrightly  taken  for  seas, 
and  were  so  called :  the  Sea  of  the  Sirens  and  the  Great 
Red  Sea.  Such  theylong  continued  to  bedeemed.  The 
seas  of  Mars  held  water  in  theory  centuries  after  the 
idea  of  the  lunar  had  vanished  into  air.  At  last,  ruthless 
science  pricked  the  pretty  bubble  analogy  had  pictured. 
Being  so  much  farther  off  than  the  Moon,  it  was  much 
later  that  their  true  character  came  out.  Come  out  it 
has,  though,  within  the  last  few  years.  Lines  —  some 
of  the  so-called  canals  —  have  been  detected  crossing 
the  seas,  lines  persistent  in  place.  This  has  effectually 
disposed  of  any  water  in  them.  But  here  again  some- 
thing of  semblance  is  left  behind.  They  are  still  the 
darkest  portions  of  the  planet,  and  their  tint  changes 
in  places  with  the  progress  of  the  planet's  year.  That 
their  color  is  that  of  vegetation,  and  that  its  change  obeys 


A   PLANET'S   HISTORY  207 

the  seasons,  stamp  it  for  vegetation  in  fact.  Thus  these 
regions  must  be  more  humid  than  the  rest  of  Mars. 
They  must,  therefore,  be  lower.  That  they  are  thus 
lower  and  possess  a  modicum  of  water  to-day  marks 
them  out  for  the  spots  where  seas  would  be,  were  there 
any  seas  to  be.  As  we  know  of  a  vera  causa  which  has 
for  ages  been  tending  to  deplete  them,  extrapolation 
from  what  is  now  going  on  returns  them  the  water  they 
have  lost  and  rehabilitates  their  ancient  aquatic  charac- 
ter. To  the  far-sight  of  inference,  seas  they  again  be- 
come in  the  morning  of  the  ages  long  ago  when  Mars 
itself  was  young. 

Nor  is  this  the  end  of  the  evidence.  When  we  com- 
pare quantitatively  the  areas  occupied  by  the  quondam 
seas  on  Mars  and  on  the  Moon,  we  find  reason  to 
increase  our  confidence  in  our  deduction.  For  the 
smaller  body,  the  Moon,  should  have  had  less  water 
relatively,  at  the  time  when  the  seas  there  were  laid 
down,  than  the  larger,  Mars.  Because  from  the  mo- 
ment its  mass  began  to  collect,  it  was  in  process  of  part- 
ing with  its  gases,  water-vapor  among  the  rest,  and,  as 
we  shall  see  more  in  detail  in  the  next  chapter,  it  had 
from  the  start  less  hold  on  them  than  Mars.  Its  oceans, 
therefore,  should  have  been  less  extensive  than  the 
Martian  ones.  This  is  what  the  present  lunar  Mare 
seem  to  attest.  They  are  less  extended  than  the  dark 
areas  of  Mars.  A  fact  which  becomes  the  more  evident 


208        THE   EVOLUTION   OF  WORLDS 

when  we  remember  that  the  Moon  has  long  turned 
the  same  face  to  the  Earth.  Her  shape,  therefore,  has 
been  that  of  an  egg,  with  the  apex  pointing  toward 
our  world.  Here  the  water  would  chiefly  collect.  The 
greater  part  of  the  seas  she  ever  had  should  be  on  our 
side  of  her  surface,  the  one  she  presents  in  perpetuity 
to  our  gaze. 

It  is  to  the  heavens  that  we  must  look  for  our  surest 
information  on  such  a  cosmic  point,  because  of  the  long 
perspective  other  bodies  give  us  of  our  own  career. 
Less  conclusive,  because  dependent  upon  less  time,  is 
any  evidence  our  globe  can  offer.  Yet  even  from  it  we 
may  learn  something;  if  nothing  else,  that  it  does  not 
contradict  the  story  of  the  sky.  To  it,  therefore,  we 
return,  quickened  in  apprehension  by  the  sights  we 
have  elsewhere  seen. 

The  first  thing  our  sharpened  sense  causes  us  to  note 
is  the  spread  of  deserts  even  within  historic  times.  Just 
as  deserts  show  by  their  latitudinal  girdling  of  the  Earth 
their  direct  dependence  upon  the  great  system  of  plane- 
tary winds,  as  meteorologists  recognizingly  call  them, 
so  a  study  of  the  fringes  of  these  belts  discloses  their  en- 
croachment upon  formerly  less  arid  lands.  The  south- 
ern borders  of  the  Mediterranean  reveal  this  all  the  way 
from  Carthage  to  Palestine.  The  disappearance  of 
their  former  peoples,  leaving  these  lands  but  scantily  in- 
habited now,  points  to  this;  because  other  regions,  as 


A   PLANET'S   HISTORY  209 

India,  which  still  retain  a  waterful  climate,  are  as  popu- 
lous as  ever.  Much  of  this  is  doubtless  due  to  the  over- 
throw of  dynasties  and  the  ensuing  lapse  of  irrigation, 
but  query:  Is  it  all  ?  For  we  have  still  more  definite 
information  in  the  drying  up  of  the  streams  which  have 
left  the  aqueducts  of  Carthage  without  continuation, 
as  much  to  water  on  the  one  hand  as  to  its  drinkers  on 
the  other.  Men  may  leave  because  of  lack  of  water, 
but  water  does  not  leave  because  of  dearth  of  men 
to  drink. 

Recent  search  around  the  Caspian  by  Huntington 
has  disclosed  the  like  degeneration  due  to  encroaching 
desertism  there.  Indeed,  it  is  no  chance  coincidence 
that  just  where  all  the  great  nations  thrived  in  the  morn- 
ing of  the  historic  times  should  be  precisely  where  popu- 
lous peoples  no  longer  exist.  For  neither  increasing  cold 
nor  increasing  heat  is  responsible  for  this,  seeing  that 
no  general  change  has  occurred  in  either.  Nor  were 
they  particularly  exposed  to  extermination  by  northern 
hordes  of  barbarians.  Egypt  as  a  world  power  died  a 
natural  death,  and  Babylonia  too;  but  the  common 
people  died  of  thirst,  indirect  and  unconscious  and  not 
wholly  of  their  own  choosing.  Prehistoric  records  make 
this  conclusion  doubly  sure,  by  lengthening  the  limit  of 
our  observation.  Both  extinct  flora  and  extinct  fauna 
tell  the  same  tale.  In  the  neighborhood  of  Cairo  petrified 
forests  attest  that  Egypt  was  not  always  a  wiped  slate, 


210        THE   EVOLUTION   OF   WORLDS 


while  the  unearthed  animals  of  the  Fayum  bear  witness 
to  water  where  no  water  is  to-day. 

Anywhere  we  wander  along   these  girdling  belts  we 
find  the  same  story  written  for  us  to  read.     The  great 


PETRIFIED    BRIDGE,  THIRD    PETRIFIED  FOREST,  NEAR  ADAMANA,  ARIZONA  — 
PHOTOGRAPH  BY  HARVEY. 

deserts  of  New  Mexico  and  Arizona  show  castellated 
structures  far  beyond  the  means  of  its  present  Indian 
population  to  inhabit.  Yet  this  retrenchment  occurred 
long  before  the  white  man  came  with  his  exterminating 
blight  on  everything  he  touched.  Nor  have  we  reason 
to  suppose  that  it  arose  in  consequence  of  invasion  by 
other  alien  hordes.  Individual  communities  may  thus 
indeed  have  perished  as  the  preservation  of  their  domi- 
ciles intact  leads  us  to  infer,  but  all  did  not  thus  vanish 
from  off  the  Earth.  Here  again  humanity  died  or 
moved  away  because  nature  dried  the  sources  of  its 


A   PLANETS   HISTORY  211 

supply.  And  here,  as  elsewhere,  we  find  prehistoric  rec- 
ord in  the  rocks  of  a  once  more  smiling  state  of  things, 
strengthening  the  testimony  we  deduce  from  man. 
The  forests,  crowning  now  only  the  greater  heights,  are 
but  the  shrinking  residues  of  what  once  clothed  the  land. 
The  well-named  Arid  Zone  is  becoming  more  so  every 
day. 

If  from  the  land  evidence  of  drying  up  we  turn  to 
the  marine,  we  see  the  same  shrinkage  at  work.  It  has 
even  been  discovered  in  a  lowering  of  the  ocean  bed,  but 
as  this  may  so  easily  be  disputed,  we  turn  to  one  aspect  of 
the  situation  which  cannot  so  easily  be  gainsaid,  —  the 
bodies  of  water  that  have  been  cut  off.  That  the  Dead 
Sea,  the  Caspian,  the  Great  Salt  Lake,  are  slowly  but 
surely  giving  way  to  land,  is  patent.  If  the  climate  at 
least  were  not  more  arid  than  before  this  could  not 
occur;  but  more  than  this,  if  the  ocean  were  not  on  the 
whole  shrinking,  there  would  be  no  tendency  to  leave 
such  arms  of  itself  behind  to  shrivel  up.  That  the 
ocean  basins  are  deepening  is  possible,  but  we  know 
of  one  depletion  which  is  not  replaced  —  evaporation 
into  space;  and  of  another  bound  to  come — withdrawal 
into  fissures  when  the  earth  shall  cease  to  be  too  hot. 

This  gradual  withdrawal  of  the  water  may  seem  an 
unpleasant  one  to  contemplate,  but  like  most  things  it 
has  its  silver  lining  in  the  hope  it  holds  out  that  some- 
time there  shall  be  no  more  sea.  Those  of  us  who 


212        THE   EVOLUTION   OF  WORLDS 

detest  the  constant  going  down  to  the  sea  in  ships  hardly 
more  than  the  occasional  going  down  with  them,  can 
take  a  crumb  of  comfort  in  the  thought.  Unfortunately 
it  partakes  of  a  somewhat  far-off  realization  in  our  dis- 
tant descendants,  coming  a  little  too  late  to  be  of  ma- 
terial advantage  to  ourselves. 

But  let  me  not  leave  the  reader  wholly  disconsolate. 
For  another  thought  we  can  take  with  us  in  closing  our 
sketch  of  so  much  of  the  Earth's  life  as  brings  it  well 
down  to  to-day,  —  the  thought  that  it  has  grown  for  us 
a  steadily  better  place  to  contemplate  from  the  earliest 
eras  to  the  present  time.  Indeed,  with  innate  prescience 
we  forbore  to  appear  till  the  prospect  did  prove  pleas- 
ing. Finally,  we  may  palliate  prognostication  by  con- 
sidering that  if  its  future  seem  a  thought  less  attractive, 
we,  at  least,  shall  not  be  there  to  see. 


CHAPTER   VIII 

DEATH    OF    A    WORLD 

EVERYTHING  around  us  on  this  Earth  we  see  is 
subject  to  one  inevitable  cycle  of  birth,  growth, 
decay.  Nothing  that  begins  but  comes  at  last  to  end. 
Not  less  is  this  true  of  the  Earth  as  a  whole  and  of  each 
of  its  sister  planets.  Though  our  own  lives  are  too  brief 
even  to  mark  the  slow  nearing  to  that  eventual  goal,  the 
past  history  of  the  Earth  written  in  its  rocks  and  the 
present  aspects  of  the  several  planets  that  circle  similarly 
round  the  Sun  alike  assure  us  of  the  course  of  aging  as 
certainly  as  if  time,  with  all  it  brings  about,  passed  in 
one  long  procession  before  our  very  eyes. 

Death  is  a  distressing  thing  to  contemplate  under  any 
circumstances,  and  not  less  so  to  a  philosopher  when 
that  of  a  whole  world  is  concerned.  To  think  that  this 
fair  globe  with  all  it  has  brought  forth  must  lapse  in 
time  to  nothingness;  that  the  generations  of  men  shall 
cease  to  be,  their  very  records  obliterated,  is  something 
to  strike  a  chill  into  the  heart  of  the  most  callous  and 
numb  endeavor  at  its  core.  That  aeons  must  roll  away 
before  that  final  day  is  to  the  mind  of  the  far-seeing 
no  consolation  for  the  end.  Not  only  that  we  shall  pass, 

213 


2i4        THE   EVOLUTION   OF  WORLDS 

but  that  everything  to  show  we  ever  were  shall  perish 
too,  seems  an  extinction  too  overpowering  for  words. 

But  vain  regret  avails  not  to  change  the  universe's 
course.  What  is  concerns  us  and  what  will  be  too. 
From  facing  it  we  cannot  turn  away.  We  may  alleviate 
its  poignancy  by  the  thought  that  our  interest  is  after 
all  remote,  affecting  chiefly  descendants  we  shall  never 
know,  and  commend  to  ourselves  the  altruistic  exam- 
ple so  nobly  set  us  by  doctors  of  medicine  who,  on  the 
demise  of  others  at  which  —  and  possibly  to  which  — 
they  have  themselves  assisted,  show  a  fortitude  not 
easily  surpassed,  a  fortitude  extending  even  to  their 
bills.  If  they  can  act  thus  unshaken  at  sight  of  their 
contemporaries,  we  should  not  fall  behind  them  in 
heroism  toward  posterity. 

Having  in  our  last  chapter  run  the  gantlet  of  the  ge- 
ologists, we  are  in  some  sort  fortified  to  face  death  — 
in  a  world  —  in  this.  The  more  so  that  we  have  some 
millenniums  of  respite  before  the  execution  of  the  de- 
cree. By  the  death  of  a  planet  we  may  designate  that 
stage  when  all  change  on  its  surface,  save  disintegration, 
ceases.  For  then  all  we  know  as  life  in  its  manifold 
manifestations  is  at  an  end.  To  this  it  may  come  by 
many  paths.  For  a  planet,  like  a  man,  is  exposed  to 
death  from  a  variety  of  untoward  events. 

Of  these  the  one  least  likely  to  occur  is  death  by  ac- 
cident. This,  celestially  speaking,  is  anything  which 


DEATH   OF   A  WORLD  215 

may  happen  to  the  solar  system  from  without,  and  is 
of  the  nature  of  an  unforeseen  catastrophe.  Our  Sun 
might,  as  we  remarked,  be  run  into.  For  so  far  as  we 
know  at  present  the  stars  are  moving  among  themselves 
without  any  too  careful  regard  for  one  another.  The 
swarm  may  be  circling  a  central  Sun  as  Andre  states, 
but  the  individual  stars  behave  more  like  the  random 
particles  of  a  gas  with  licensed  freedom  to  collide; 
whereas  we  may  liken  the  members  of  the  solar  system 
to  molecules  in  the  solid  state  held  to  a  centre  from 
which  they  can  never  greatly  depart.  Their  motions 
thus  afford  a  sense  of  security  lacking  in  the  universe 
at  large. 

Such  an  accident,  a  collision  actual  or  virtual  with 
another  sun,  would  probably  occur  with  some  dark  star; 
of  which  we  sketched  the  ultimate  results  in  our  first 
chapter.  The  immediate  ones  would  be  of  a  most  dis- 
astrous kind.  For  prefatory  to  the  new  birth  would 
be  the  dissolution  to  make  such  resurrection  possible. 
Destruction  might  come  direct,  or  indirectly  through 
the  Sun.  For  though  the  Sun  would  be  the  tramp's 
objective  point,  we  might  inadvertently  find  ourselves 
in  the  way.  The  choice  would  be  purely  academic ;  be- 
tween being  powdered,  or  deorbited  and  burnt  up. 

So  remote  is  this  contingency  that  it  need  cause  us 
no  immediate  alarm,  as  I  carefully  pointed  out.  But  so 
strong  is  the  instinct  of  self-preservation  and  so  pleasur- 


216        THE   EVOLUTION   OF   WORLDS 

able  the  sensation  of  spreading  appalling  news,  that  the 
press  of  America,  and  incidentally  Europe,  took  fire,  with 
the  result,  so  I  have  been  written,  that  by  the  time  the 
pictured  catastrophe  reached  the  Pacific  "it  had  as- 
sumed the  dimensions  of  a  first  magnitude  fact." 

This  is  the  first  way  in  which  our  world  may  come  by 
its  death.  It  is  possible,  but  unlikely.  For  our  Earth, 
long  before  that,  is  morally  certain  to  perish  otherwise. 

The  second  mode  is  one,  incident  to  the  very  consti- 
tution of  our  solar  system.  It  follows  as  a  direct  out- 
come of  that  system's  mechanical  evolution,  and  may 
be  properly  designated,  therefore,  as  due  to  natural 
causes.  It  might  be  diagnosed  as  death  by  paralysis. 
For  such  it  resembles  in  human  beings,  palsy  of  indi- 
vidual movement  afflicting  a  planet  instead  of  a  man. 

Tidal  friction  is  the  slow  undermining  cause;  a  force 
which  is  constantly  at  work  in  the  action  of  every  body 
in  the  universe  upon  every  other.  As  we  previously 
explained,  the  pull  of  one  mass  upon  another  is  inevi- 
tably differential.  Not  only  is  the  second  drawn  in  its 
entirety  toward  the  first,  falling  literally  as  it  circles 
round,  but  the  nearer  parts  are  drawn  more  than  the 
centre  and  the  centre  more  than  those  farthest  away. 
We  may  liken  the  result  to  a  stretched  rotating  rubber 
ball,  with,  however,  one  important  difference, — that  each 
layer  is  more  or  less  free  to  shear  over  the  others.  The 
bulge,  solicited  by  the  rotation  to  keep  up,  by  the  dis- 


DEATH   OF   A  WORLD  217 

turber  to  lag  behind,  is  torn  two  ways,  and  the  friction 
acts  as  a  break  upon  the  body's  rotation,  tending  first  to 
turn  it  over  if  it  be  rotating  backward  and  then  to  slow 
it  down  till  the  body  presents  the  same  face  in  perpetuity 
to  its  primary.  The  tides  are  the  bulge,  not  simply 
those  superficial  ones  which  we  observe  in  our  oceans, 
and  know  to  be  so  strong,  but  substantial  ones  of  the 
whole  body  which  we  must  conceive  thus  as  egg-shaped 
through  the  action  that  goes  on  —  the  long  diameter 
of  the  egg  pointing  somewhat  ahead  of  the  line  joining 
its  centre  to  the  distorting  mass.  All  the  bodies  in  the 
solar  system  are  thus  really  egg-shaped,  though  the  de- 
formation is  so  slight  as  to  escape  detection  observa- 
tionally.  The  knowledge  is  an  instance  of  how  much 
more  perceptive  the  brain  is  than  the  eye.  For  we  are 
certain  of  the  fact,  and  yet  to  see  it  with  our  present 
means  is  impossible,  and  may  long  remain  so. 

Two  concomitant  symptoms  follow  the  friction  of  the 
tidal  ansae :  a  shift  of  the  plane  in  which  the  rotation 
takes  place,  and  a  loss  of  speed  in  the  spin  itself.  The 
first  tends  to  bring  the  plane  of  rotation  down  to  the 
orbital  plane,  with  rotation  and  revolution  in  the  same 
sense.  This  effect  takes  place  quicker  than  the  other, 
and  in  consequence  different  stages  may  be  noted  in  the 
creeping  paralysis  by  which  the  body  is  finally  overcome. 
Loss  of  seasons  characterizes  the  first.  For  the  coinci- 
dence of  the  two  planes  means  invariability  in  the  Sun's 


2i 8        THE   EVOLUTION   OF   WORLDS 

declination  throughout  the  year  for  a  given  latitude. 
This  reduces  all  its  days  to  one  dead  level  in  which 
summer  and  winter,  spring  and  autumn,  are  always  and 
everywhere  the  same.  There  is  thus  a  return  at  the  end 
of  the  planet's  career  to  an  uneventful  condition  rem- 
iniscent of  its  start;  a  senility  in  planets  comparable 
to  second  childhood  in  man. 

In  large  planets  this  outgrowing  of  seasons  occurs 
before  they  have  any,  while  the  planet  is  yet  cloud- 
wrapped.  Such  planets  know  nothing  of  some  attri- 
butes of  youth,  like  those  unfortunate  men  who  never 
were  boys;  just  as  reversely  the  meteorites  are  boys 
that  never  grew  up.  For  if  the  planet  be  large,  the  ac- 
tion of  the  tidal  forces  is  proportionately  more  power- 
ful; while  on  the  other  hand  the  self-aging  of  the 
planet  is  greatly  prolonged,  and  thus  it  may  come  about 
that  the  former  process  outstrips  the  latter  to  the  missing 
of  seasons  entirely.  This  is  sure  to  be  the  case  with 
Jupiter,  as  the  equator  has  already  got  down  to  within 
3°  of  the  orbit,  and  threatens  to  be  the  case  with  Saturn. 
These  bodies,  then,  when  they  shall  have  put  off  their 
swaddling  clothes  of  cloud,  will  wake  to  climates  without 
seasons;  globes  where  conditions  are  always  the  same 
on  the  same  belts  of  latitude,  and  on  which  these  alter 
progressively  from  equator  to  pole.  Variety  other  than 
diurnal  is  thus  excluded  from  their  surfaces  and  from 
their  skies.  For  the  Sun  and  stars  will  rise  always  the 


DEATH   OF   A  WORLD  219 

same,  in  punctual  obedience  only  to  the  slowly  shifting 
year. 

The  next  stage  of  deprivation  is  the  parting  with  the 
day.  Although  the  day  disappears,  the  result  is  too 
much  day  or  too  little,  depending  on  where  you  choose 
to  consider  yourself  upon  the  afflicted  orb.  For 
tidal  friction  proceeds  to  lengthen  the  twenty-four  or 
other  hours  first  to  weeks,  then  months,  then  years,  and 
at  last  to  infinity;  thus  bringing  the  sun  to  a  stock-still 
on  the  meridian,  to  flood  one  side  of  the  world  with 
perpetual  day  and  plunge  the  other  in  eternal  night. 

Which  of  these  two  hemispheres  would  be  the  worse 
abode,  is  matter  of  personal  predilection;  dust  or 
glacier,  deserts  both.  Everlasting  unshielded  noon 
would  cause  a  wind  circulation  from  all  points  of  the 
enlightened  periphery  to  the  centre,  whence  a  funnel- 
shaped  current  would  rise  to  overflow  back  into  the 
antipodes,  thence  to  return  by  the  horizon  again.  As 
the  night  side  would  be  several  hundred  degrees  at  least 
colder  than  the  noon  one,  all  the  moisture  would  be 
evaporated  on  the  sunlit  hemisphere,  to  be  carried  round 
and  deposited  as  ice  on  the  other,  there  to  stay.  Life 
would  be  either  toasted  or  frappe.  A  Sahara  backed 
by  polar  regions  would  be  the  obverse  and  the  reverse 
of  the  shield. 

The  reader  may  deem  the  picture  a  fancy  sketch 
which  possibly  may  not  appeal  to  him.  Nevertheless, 


220        THE   EVOLUTION   OF   WORLDS 

it  not  only  is  possible,  but  one  which  has  overtaken  our 
nearest  of  neighbors.  To  this  pass  the  Mater  Amorum, 
Venus  herself,  has  already  been  brought.  She  betrays 
it  by  the  wrinkles  which  modern  observation  has  re- 


October  15,  1896.  February  12,  1897.  March  26,  1897. 

VENUS  —  DRAWINGS  BY  DR.  LOWELL  SHOWING  AGREEMENT  AT  DIFFERENT 

DISTANCES. 

vealed  upon  her  face.  Innocent  critics,  with  a  gallantry 
one  would  hardly  have  credited  them,  —  which  shows 
how  one  may  wrong  even  the  humblest  of  creatures,  — 
have  denied  the  existence  of  these  marks  of  age,  on  the 
chivalrous  a  priori  assumption  that  it  could  not  possi- 
bly be  true  because  never  seen  before.  Their  negation, 
in  naive  ignorance  of  the  facts,  partakes  the  logic 
of  the  gallant  captain,  who,  when  asked  by  a  lady 
to  guess  her  age,  replied  :  "Ton  my  word,  I  haven't  the 


DEATH   OF   A  WORLD  221 

slightest  idea,"  hastily  adding,  "  But  you  don't  look  it !" 
Less  commendable  than  this  conventional  nescience, 
but  unfortunately  more  to  the  point,  is  the  evidence  of 
prying  scientific  curiosity.  Shrewdly  divined  as  much 
as  detected  by  Schiaparelli,  made  more  certain  by  the 
crow's-feet  disclosed  at  Flagstaff,  and  corroborated  by 
the  testimony  of  the  spectroscope  there,  her  isochronism 
of  rotation  and  revolution  lies  beyond  a  doubt.  At- 
traction to  her  lord  has  conquered  at  last  her  who  was 
the  cynosure  of  all.  Venus,  in  her  old  age,  stares  for- 
ever at  the  Sun,  and  we  all  know  how  ill  an  aging 
beauty  can  support  a  garish  light. 

Mercury  has  been  brought  to  a  like  pass.  This  was 
evident  even  before  the  facts  came  out  about  Venus,  for 
Venus,  true  to  her  instincts,  shields  herself  with  a  veil 
of  air  which  largely  baffles  man's  too  curious  gaze. 
Mercury,  on  the  other  hand,  offers  no  objection  to  ob- 
servation. When  looked  for  at  the  proper  time,  his 
markings  are  quite  distinct,  dark,  broken  lines  suggesting 
cracks.  Schiaparelli,  again,  was  the  first  to  perceive 
the  true  state  of  the  case,  and  his  observations  were  in- 
dependently confirmed  and  extended  at  Flagstaff  in 
1896.  In  so  doing  the  latter  disclosed  a  very  interesting 
fact.  It  was  evident  that  the  markings  held  in  general 
a  definite  fixed  position  upon  the  illuminated  part  of 
the  disk,  showing  that  the  planet  kept  the  same  face 
always  to  the  Sun.  But  systematic  observation,  con- 


222        THE   EVOLUTION   OF  WORLDS 

tinued  day  after  day  for  weeks,  disclosed  a  curious  shift, 
which,  though  slight,  was  unmistakable.  Upon  thought 
the  cause  suggested  itself,  and  on  being  subjected  to 
calculation  proved  equal  to  such  accounting.  In  this 


DIAGRAM  OF  LIBRATION  IN  LONGITUDE  DUE  TO  ROTATION. 

singular  systematic  sway  stood  revealed  the  libration 
in  longitude  caused  by  the  eccentricity  of  the  planet's 
orbit. 

Mercury  revolves  about  the  Sun  in  an  ellipse  more 
eccentric  than  that  of  any  other  principal  planet.  At 
times  he  is  half  as  far  ofl  again  from  him  as  he  is  at 
others.  When  near,  he  travels  faster  than  when  far. 


DEATH   OF   A  WORLD  223 

For  both  reasons,  nearness  and  speed,  his  angular 
revolution  about  the  Sun  varies  greatly  from  point  to 
point  according  to  where  he  finds  himself  in  his  orbit. 
His  rotation,  however,  is  necessarily  uniform.  For  even 
the  Sun  has  no  power  at  once  to  change  the  enormous 
moment  of  momentum  of  his  axial  spin.  In  conse- 
quence, at  times  his  angular  velocity  of  revolution 
gains  on  his  rotation,  at  other  times  loses,  both  coming 
out  together  at  the  end  of  a  complete  Mercurial  year. 
The  result  is  a  superb  rhythmic  oscillation,  a  true 
mercurial  pendulum  compensated  by  celestial  laws  to 
perfect  isochronism  of  swing. 

The  outward  sign  of  this  shows  in  the  movement  of 
the  markings.  To  observers  in  space  like  ourselves, 
the  planet  seems  to  sway  his  head  as  he  travels  along 
his  orbit.  For  weeks  he  turns  his  face,  as  shown  by  the 
markings  on  it,  more  and  more  over  to  the  left;  then 
turns  it  back  again  as  far  over  to  the  right.  It  is  as  if 
he  were  looking  furtively  around  as  he  hastens  over  his 
planetary  path. 

Venus,  of  course,  is  equally  subject  to  this  law  of 
distraction,  but  owing  to  the  almost  perfect  circularity 
of  her  orbit  she  is  less  visibly  affected.  In  fact,  it  is  not 
possible  to  detect  her  lapse  from  a  fixed  regard  to  the 
Sun.  At  most  it  is  no  more  than  a  glance  out  of  the 
corner  of  her  eyes  —  her  slight  deviation  from  perfect 
rectitude  of  demeanor.  Knowledge  of  the  laws  gov- 


224       THE  EVOLUTION  OF  WORLDS 

erning  such  action  alone  permits  us  to  recognize  its 
occurrence. 

Mercury  and  Venus  are  the  only  planets  as  yet  that 
turn  a  constant  face  to  their  overruling  lord.  The 
reason  for  this  appears  when  one  goes  into  the  matter 
analytically.  The  tidal  force  is  not  the  direct  pull  of 
the  Sun  on  a  particle  of  the  body,  but  the  difference  in 
the  pulls  upon  a  particle  at  the  centre  and  one  at  the 
circumference.  Being  differential,  it  depends  directly 
upon  the  radius  of  the  distorted  body  and  inversely  upon 
the  third  power  of  its  distance  away.  As  the  space 
through  which  the  force  acts  is  proportional  to  the  force 
itself,  the  effect  is  as  the  squares  of  the  quantities 
mentioned,  or,  inversely,  as  the  sixth  power  of  the 
distance  and  as  the  square  of  the  body's  radius.  The 
result  thus  proves  greatest  on  the  planets  nearest  to  the 
Sun,  and  diminishes  rapidly  as  we  pass  outward  from 
him.  If,  then,  the  solar  force  had  had  time  enough  to 
produce  its  effects,  it  would  be  first  in  Mercury  and  then 
in  Venus  that  it  should  be  seen.  And  this  is  precisely 
where  we  observe  it. 

The  Moon  presents  us  a  well-known  case  of  such  filial 
regard,  resulting  in  permanent  incompetency  of  action 
on  its  own  account.  It  turns  always  the  same  face  to 
us,  following  us  about  with  the  mute  attention  of  a  dog 
to  its  master.  Here  again  the  libration  may  be  detected, 
for  no  dog  but  makes  excursions  on  the  road.  This  case 


DEATH   OF   A  WORLD  225 

differs  from  those  of  Mercury  and  Venus  in  that  the  body 
to  which  the  regard  is  paid  is  not  also  the  dispenser  of 
light  and  warmth.  In  consequence,  though  the  side  of 
the  Moon  with  which  we  are  presented  remains  always 


MOON FULL  AND  HALF,   PHOTOGRAPHED  AT  THE  LOWELL   OBSERVATORY. 

the  same,  we  do  not  always  see  it;  the  light  creeping 
over  it  with  the  progress  of  the  lunation,  from  new  to 
full.  On  this  account  the  worst  that  happens  to  our 
Moon  in  its  old  age  is  that  its  day  becomes  its  month. 
Our  Moon  is  not  peculiar  in  having  its  day  and  its 
month  the  same.  On  the  contrary,  it  is  now  the  rule 
with  satellites  thus  to  protract  their  days.  So  far  as  we 
can  observe,  all  the  large  satellites  of  Jupiter  turn  the 
same  face  to  him;  those  of  Saturn  pay  him  a  like  re- 
gard; while  about  those  of  Uranus  and  Neptune  we 
are  too  far  off  to  tell.  Their  direct  respect  for  their 
primary,  with  only  secondary  recognition  of  the  Sun, 
keeps  them  from  the  full  consequences  of  their  fatal 


226        THE   EVOLUTION   OF   WORLDS 

yielding  to  attraction.  It  is  bad  enough  to  have  the 
day  half  a  month  long,  but  worse  to  have  one  that  never 
ends,  or,  still  worse,  perpetual  night. 

In  our  diagnosis  of  the  cause  of  death  in  planets,  we 
now  pass  from  paralysis  to  heart  failure.  For  so  we 
may  speak  of  the  next  affection  which  ends  in  their 
taking  off,  since  it  is  due  to  want  of  circulation  and  lack 
of  breath.  It  comes  of  a  planet's  losing  first  its  oceans 
and  then  its  air. 

To  understand  how  this  distressing  condition  comes 
about,  we  must  consider  one  of  the  interesting  scientific 
legacies  of  the  nineteenth  century  to  the  twentieth :  the 
kinetic  theory  of  gases. 

The  kinetic  theory  of  gases  supposes  them  to  be  made 
up  of  minute  particles  all  alike,  which  are  perfectly 
elastic  and  are  travelling  hither  and  thither  at  great 
speeds  in  practically  straight  lines.  In  consequence, 
these  are  forever  colliding  among  themselves,  giving 
and  taking  velocities  with  bewildering  rapidity,  resulting 
in  a  state  of  confusion  calculated  to  drive  a  computer 
mad.  Somebody  has  likened  a  quiet  bit  of  air  to  a 
boiler  full  of  furious  bees  madly  bent  on  getting  out. 
The  simile  flatters  the  bees.  To  follow  the  vicissi- 
tudes of  any  one  molecule  in  this  hurly-burly  would 
be  out  of  the  question ;  still  more,  it  would  seem,  that  of 
all  of  them  at  once.  Yet  no  less  Herculean  a  task  con- 
fronts us.  To  find  out  about  their  motions,  we  are 


DEATH   OF   A  WORLD  227 

therefore  driven  to  what  is  called  the  statistical  method 
of  inquiry,  —  which  is  simply  a  branch  of  the  doctrine 
of  probabilities.  It  is  the  method  by  which  we  learn 
how  many  people  are  going  to  catch  cold  in  Boston  next 


ILLUSTRATING  MOLECULAR  MOTION  IN  A  GAS  (BLACK  MOLECULES  HERE  CON- 
SIDERED AT  REST). 

week  when  we  know  nothing  about  the  people,  or  about 
colds,  or  about  catching  them.  At  first  sight  it  might 
seem  as  if  we  could  never  discover  anything  in  this 
hopelessly  ignorant  way,  and  as  if  we  had  almost  better 
call  in  a  doctor.  But  in  the  multitude  of  colds  —  not 
of  counsellors  —  lies  wisdom.  So  in  other  things  not 
hygienic.  As  you  cannot  possibly  divine,  for  instance, 
what  each  boy  in  town  is  going  to  do  during  the  year, 


228        THE   EVOLUTION   OF   WORLDS 

nor  what  is  his  make  of  mind,  how  can  you  say  whether 
he  will  accidentally  discharge  a  firearm  and  shoot  his 
playmate  or  not !  And  yet  if  you  take  all  the  boys  of 
Boston,  you  can  predict  to  a  nicety  how  many  will  thus 
let  off  a  gun  and  "  not  know  that  it  was  loaded." 

In  this  only  genuine  method  of  prophecy,  complete 
ignorance  of  all  the  actual  facts,  we  are  able  without 
knowing  anything  whatever  about  each  of  the  mole- 
cules to  predicate  a  good  deal  about  them  all.  To  begin 
with,  the  pressure  a  gas  exerts  upon  the  sides  of  a  vessel 
containing  it  must  be  the  bombardment  the  sides  re- 
ceive from  the  little  molecules;  and  the  heating  due 
this  rain  of  blows,  or  the  temperature  to  which  the  ves- 
sel is  raised,  must  measure  their  energy  of  translation. 
On  this  supposition  it  is  found  that  the  laws  of  Avoga- 
dro  and  of  Boyle  are  perfectly  accounted  for,  besides 
many  more  properties  of  gases  which  the  theory  ex- 
plains, and  as  nothing  yet  has  been  encountered  seri- 
ously contradicting  it,  we  may  consider  it  as  almost  as 
surely  correct  as  the  theory  of  gravitation.  To  three 
great  geniuses  of  the  last  century  we  owe  this  remarkable 
discovery  —  Clausius,  Clerk  Maxwell,  and  Boltzmann. 

By  determining  the  density  of  a  gas  at  a  given  tem- 
perature and  under  a  given  pressure,  we  can  find  by  the 
statistical  method  the  average  speed  of  its  molecules. 
It  depends  on  the  most  probable  distribution  of  their 
energy.  For  hydrogen  at  the  temperature  of  melting 


DEATH   OF   A  WORLD 


229 


ice,  and  under  atmospheric  pressure,  this  speed  proves 
to  be  a  little  over  a  mile  a  second  —  a  speed,  curiously 
enough,  which  is  to  that  of  light  almost  exactly  as  centi- 
metres to  miles.  But  some  of  the  molecules  are  going 


DISTRIBUTION  OF  MOLECULAR  VELOCITIES  IN  A  GAS. 

at  speeds  much  above  the  mean;  fewer  and  fewer  as 
the  speed  gets  higher.  Just  how  many  there  are  for 
any  assigned  speed,  we  can  calculate  by  the  same  in- 
genious application  of  unknown  quantities. 

These  speeds  have  been  found  for  a  temperature  of 
freezing,  and  as  the  speed  varies  as  the  square  root  of 
the  absolute  temperature,  we  might  suppose  that  when 
an  adventurous  or  lucky  molecule  arrived  at  practically 
the  limit  of  the  atmosphere,  where  the  cold  is  intense,  it 
would  become  numbly  sluggish.  But  let  us  consider 


230        THE   EVOLUTION   OF   WORLDS 

this.  When  we  enclose  a  gas  in  a  cooler  vessel,  the 
molecules  bombard  the  sides  more  than  they  are  bom- 
barded back.  In  consequence,  they  lose  energy;  as  we 
say,  are  cooled.  But  in  free  air  if  a  molecule  be  fortunate 
enough  to  elude  its  neighbors,  there  is  nothing  to  take 
away  its  motion  but  the  ether  through  radiation,  and 
this  is  a  very  slow  process.  Thus  the  escaping  fugitive 
must  arrive  at  the  confines  of  the  air  with  the  speed  it 
had  at  its  last  encounter.  We  reach,  then,  this  result: 
In  space  there  is  no  such  thing  as  temperature;  tem- 
perature being  simply  the  aggregate  effect  of  molecular 
temperament.  The  reason  we  should  consider  it  un- 
commonly cold  up  there  is  that  fewer  molecules  would 
strike  us.  Quantity,  therefore,  in  our  estimation  re- 
places quality,  —  a  possible  substitution  which  also  ac- 
counts for  some  reputations,  literary  or  otherwise.  The 
only  forces  which  could  affect  this  lonely  molecule  would 
be  the  heating  by  the  Sun,  the  repellent  force  of  light, 
and  gravity. 

Now  the  speed  which  gravity  on  the  Earth  can  con- 
trol is  6.9  miles  a  second.  It  can  impart  this  to  a  body 
falling  freely  to  it  from  infinite  space,  and  can  therefore 
annul  it  on  the  way  up,  and  no  more.  If,  then,  any  of 
the  molecules  reach  the  outer  boundary  of  the  air  going 
at  more  than  this  speed,  they  will  pass  beyond  the 
Earth's  power  to  restrain.  They  will  become  little 
rovers  in  space  on  their  own  account,  and  dart  off  on 


DEATH   OF   A  WORLD  231 

interstellar  travels  of  their  own.  This  extension  of  the 
kinetic  theory  and  of  the  consequent  voyages  of  the 
molecules  is  due  to  Dr.  Johnstone  Stoney,  who  has 
since,  humorously  enough,  tried  to  stop  the  very  balls 
he  set  rolling.  First  thoughts  are  usually  the  best,  after 
all. 

As  among  the  molecules  some  are  already  travelling 
at  speeds  in  excess  of  this  critical  velocity,  molecules 
must  constantly  be  attaining  to  this  emancipation,  and 
thus  be  leaving  the  Earth  for  good.  In  consequence 
there  is  a  steady  drain  upon  its  gaseous  covering. 
Furthermore,  as  we  know  from  comets'  tails,  the  re- 
pellent power  of  the  light-waves,  what  we  may  call  the 
levity  of  light,  much  exceeds  upon  such  volatile  va- 
grants the  heat  excitement  or  even  the  gravity  of  the 
Sun,  so  that  we  arrive  at  this  interesting  conclusion  — 
their  escape  is  best  effected  under  cover  of  the  night. 

Again,  the  heavier  the  gas,  the  less  its  molecular  speed 
at  a  given  temperature,  because  its  kinetic  energy  which 
measures  that  temperature  is  one-half  the  molecule's 
mass  into  the  square  of  its  speed.  Thus  their  ponder- 
osity prevents  as  many  of  them  from  following  their 
more  agile  cousins  of  a  different  constitution.  So  that 
the  lighter  gases  are  sooner  gone.  Water-vapor  leaves 
before  oxygen.  Nor  is  there  any  escape  from  this  es- 
cape of  the  gases.  It  may  take  excessively  long,  but 
go  they  must  until  a  solitary  individual  who  happens 


232        THE   EVOLUTION   OF   WORLDS 

to  have  had  the  wrong  end  of  the  last  collision  is  alone 
left  hopelessly  behind. 

Another  factor  also  is  concerned.  The  smaller  the 
planet,  the  lower  the  utmost  velocity  it  can  control,  and 
the  quicker,  therefore,  it  must  lose  its  atmosphere.  For 
a  greater  number  of  molecules  must  at  every  instant 
reach  the  releasing  speed.  Thus  those  bodies  that 
are  little  shall,  perforce,  have  less  to  cover  themselves 
withal. 

Now  this  inevitable  depletion  of  their  atmospheric 
envelopes,  the  aspects  of  the  various  planets  strikingly 
attest.  They  do  so  in  most  exemplary  fashion,  accord- 
ing to  law.  The  larger,  the  major  planets,  as  we  have  al- 
ready remarked,  have  a  perfect  plethora  of  atmosphere, 
more  than  we  at  least  know  what  to  do  with  in  the  way 
of.  cataloguing  yet.  The  medium-sized,  like  our  own 
Earth,  have  a  very  comfortable  amount;  Mars,  an  un- 
comfortable one,  as  we  consider,  and  the  smallest  none 
at  all.  All  the  smaller  bodies  of  our  system  are  thus 
painfully  deprived  so  far  as  we  can  discover.  We  are 
certain  of  it  in  the  case  of  our  Moon  and  Mercury, 
the  only  ones  we  can  see  well  enough  to  be  sure.  In 
further  evidence  it  has  been  shown  at  the  Yerkes  and  at 
Flagstaff  that  no  perceptible  effect  of  air  betrays  itself 
in  the  spectroscopic  imprint  of  the  rings  of  Saturn,  those 
tiny  satellites  of  his,  and  very  recently  a  spectrogram  of 
Ganymede,  Jupiter's  third  moon,  made  at  Flagstaff  for 


DEATH   OF   A  WORLD  233 

the  purpose  by  Mr.  E.  C.  Slipher  has  proved  equally 
void  of  atmospheric  hint. 

With  the  loss  of  water  and  of  air,  all  possibility  of 
development  departs.  Not  only  must  every  organism 
die,  but  even  the  inorganic  can  no  longer  change  its 
state.  In  the  extinction  thus  not  only  of  inhabitants 
but  of  the  habitat  that  made  them  possible,  occurs  a 
curious  inversion  of  the  order  we  are  familiar  with  in 
the  life-history  of  organisms.  In  planets  it  is  the 
grandchildren  that  die  first,  then  the  children,  and 
lastly  their  surviving  parent.  And  this  is  not  acci- 
dental, but  inevitably  consequent  upon  their  respective 
origins.  For  the  off-spring,  as  we  may  spell  it  with  a 
hyphen,  of  any  cosmic  mass  is  of  necessity  smaller  than 
that  from  which  it  issued.  Being  smaller,  it  must  age 
quicker.  In  the  natural  order  of  events,  then,  its  end 
must  be  reached  first. 

Such  has  been  the  course  taken,  or  still  taking,  by  the 
bodies  of  our  solar  family.  The  latest  generation  has 
already  succumbed  to  this  ebbing  of  vitality  with  time. 
Every  one  of  the  satellites  of  the  planets  —  those  of 
Neptune,  Uranus,  Saturn,  Jupiter,  and  our  own  Moon — 
is  practically  dead;  born  so  the  smaller  which  never 
were  alive.  Our  own  Moon  carries  its  decrepitude  on 
its  face.  To  all  intents  and  purposes  its  life  is  past; 
and  that  it  had  at  one  time  a  very  fiery  existence,  the 
great  lunar  craters  amply  testify.  It  is  now,  for  all  its 


234        THE   EVOLUTION   OF  WORLDS 

flooding  with  radiance  our  winter  nights,  the  lifeless 
statue  of  its  former  self. 

The  same  inevitable  end,  in  default  of  others,  is 
now  overtaking  the  planetary  group.  Its  approach  is 
stamped  on  the  face  of  Mars.  There  we  see  a  world 
dying  of  exhaustion.  The  signs  of  it  are  legible  in  the 
markings  we  descry.  How  long  before  its  work  is  done, 
we  ignore.  But  that  it  is  a  matter  of  time  only,  our 
study  of  the  laws  of  the  inexorable  lead  us  to  conclude. 
Mars  has  been  spared  the  fate  of  Mercury  and  Venus 
to  perish  by  this  other  form  of  planetary  death. 

Last  in  our  enumeration  of  the  causes  by  which  the 
end  of  a  world  may  be  brought  about,  because  the  last 
to  occur  in  order  of  time,  is  the  extinction  of  the  Sun 
itself.  Certain  to  come  and  conclude  the  solar  system's 
history  as  the  abode  of  life,  if  all  the  others  should  by 
any  chance  fail  to  precede  it,  it  fittingly  forms  the  climax, 
grand  in  its  very  quietude,  of  all  that  went  before. 

By  the  same  physical  laws  that  caused  our  Earth 
once  to  be  hot,  the  Sun  shines  to-day.  Only  its  greater 
size  has  given  it  a  life  and  a  brilliancy  denied  to  smaller 
orbs.  The  falling  together  of  the  scattered  particles  of 
which  it  is  composed,  caused,  and  still  is  causing,  the 
dazzling  splendor  it  emits.  And  so  long  as  it  remains 
gaseous,  its  temperature  must  increase,  in  spite  of  its 
lavish  expenditure  of  heat,  as  Homer  Lane  discovered 
forty  years  ago. 


DEATH   OF   A  WORLD  235 

But  the  Sun's  store  of  heat,  immense  as  it  is  to-day, 
and  continued  as  it  is  bound  to  be  for  untold  aeons  by 
means  of  contraction  of  its  globe  upon  itself,  and  pos- 
sibly by  other  causes,  must  some  day  give  out.  From  its 
present  gaseous  condition  it  must  gradually  but  event- 
ually contract  to  a  solid  one,  and  this  in  turn  radiate 
all  its  heat  into  space.  Slowly  its  lustre  must  dim  as 
it  becomes  incapable  of  replenishing  its  supply  of 
motive  power  by  further  shrinkage  in  size.  Fitfully, 
probably,  like  Mira  Ceti  to-day,  it  will  show  tem- 
porary bursts  of  splendor  as  if  striving  to  regain  the 
brightness  it  had  lost,  only  to  sink  after  each  effort 
into  more  and  more  impotent  senility.  At  last  some 
day  must  come,  if  we  may  talk  of  days  at  all  when 
the  great  event  occurs  when  all  days  shall  be  blotted 
out,  that  the  last  flicker  shall  grow  extinct  in  the  orb 
that  for  so  long  has  made  the  hearth  of  the  whole 
system.  For,  presciently  enough,  the  Latin  word  focus 
means  hearth,  and  the  body  which  includes  within  it  the 
focus  about  which  all  the  planets  revolve  also  con- 
stitutes the  hearth  from  which  they  all  are  lighted  and 
warmed. 

When  this  ultimate  moment  arrives  and  the  last  spark 
of  solar  energy  goes  out,  the  Sun  will  have  reverted  once 
more  to  what  it  was  when  the  cataclysm  of  the  foretime 
stranger  awoke  it  into  activity.  It  will  again  be  the 
dark  body  it  was  when  our  peering  into  the  past  first 


236        THE   EVOLUTION   OF  WORLDS 

descries  it  down  the  far  vista  of  unrecorded  time. 
Ghostlike  it  will  travel  through  space,  unknown,  un- 
heralded, till  another  collision  shall  cause  it  to  take  a 
place  again  among  the  bright  company  of  heaven. 
Thus,  in  our  account  of  the  career  of  a  solar  system,  we 
began  by  seeing  with  the  mind's  eye  a  dark  body  travel- 
ling incognito  in  space,  and  a  dark  body  we  find  our- 
selves again  contemplating  at  the  end. 

In  this  kaleidoscopic  biograph  of  the  solar  system's 
life,  each  picture  dissolves  into  its  successor  by  the  falling 
together  of  its  parts  to  fresh  adjustments  of  stability,  as 
in  that  instrument  of  pleasure  which  so  witched  our 
childish  wonder  in  early  youth.  Just  as  when  a  com- 
bination had  proved  so  pretty,  once  gone,  to  our  sor- 
row no  turning  of  the  handle  could  ever  bring  it  back, 
so  in  the  march  of  worlds  no  retrace  is  possible  of  steps 
that  once  are  past.  Inexorable  permutations  lead  from 
one  state  to  the  next,  till  the  last  of  all  be  reached. 

Yet,  unlike  our  childhood's  toy,  reasoning  can  conjure 
up  beside  the  present  picture  far  vistas  of  what  pre- 
ceded it  and  of  what  is  yet  to  come.  Hidden  from 
thought  only  by  the  distraction  of  the  day,  as  the  uni- 
verse to  sight  lies  hid  by  the  day's  overpowering  glare, 
both  come  out  on  its  withdrawal  till  we  wonder  we  never 
gazed  before.  Our  own  surroundings  shut  out  the 
glories  that  lie  beyond.  Our  veil  of  atmosphere  cloaks 
them  from  our  view.  But  wait,  as  an  astronomer,  till 


DEATH   OF   A  WORLD  237 

the  Sun  sinks  behind  the  hills  and  his  gorgeous  gold  of 
parting  fades  to  amber  amid  the  tender  tapestry  of  trees. 
The  very  air  takes  on  a  meaning  which  the  flood  of  day 
had  swamped.  Seen  itself,  no  longer  imperfectly  seen 
through,  it  wakes  to  semi-sentient  existence,  a  spirit 
come  to  life  aloft  to  shield  us  from  the  too  immediate 
vacancy  of  space.  The  perfumes  of  the  soil,  the  trees, 
the  flowers,  steal  out  to  it,  as  the  twilight  glow  itself  ex- 
hales to  heaven.  In  the  hushed  quiet  of  the  gloaming 
Earth  holds  her  breath,  prescient  of  a  revelation 
to  come. 

Then  as  the  half-light  deepens,  the  universe  appears. 
One  by  one  the  company  of  heaven  stand  forth  to 
human  sight.  Venus  first  in  all  her  glory  brightens 
amid  the  dying  splendor  of  the  west,  growing  in  lustre 
as  her  setting  fades.  From  mid-heaven  the  Moon  lets 
fall  a  sheen  of  silvery  light,  the  ghostly  mantle  of  her 
ghostlike  self,  over  the  silent  Earth.  Eastward  Jupiter, 
like  some  great  lantern  of  the  system's  central  sweep, 
swings  upward  from  the  twilight  bow  to  take  possession 
of  the  night.  Beyond  lies  Saturn,  or  Uranus  perchance 
dim  with  distance,  measuring  still  greater  span.  All  in 
order  in  their  several  place  the  noble  cortege  of  the  Sun 
is  exposed  to  view,  seen  now  by  the  courtesy  of  his  with- 
drawal, backgrounded  against  the  immensity  of  space. 
Great  worlds,  these  separate  attendants,  and  yet  as  noth- 
ings in  the  void  where  stare  the  silent  stars,  huge  suns 


238        THE   EVOLUTION   OF   WORLDS 

themselves  with  retinues  unseen,  so  vast  the  distances 
'twixt  us  and  them. 

No  less  a  revelation  awaits  the  opening  of  the  shut- 
ters of  the  mind.  If  night  discloses  glimpses  of  the 
great  beyond,  knowledge  invests  it  with  a  meaning  un- 
folding and  extending  as  acquaintance  grows.  Sight 
is  human ;  insight  seems  divine.  To  know  those  points 
of  light  for  other  worlds  themselves,  worlds  the  tele- 
scope approaches  as  the  years  advance,  while  study 
reconstructs  their  past  and  visions  forth  their  future, 
is  to  be  made  free  of  the  heritage  of  heaven.  Time 
opens  to  us  as  space  expands.  We  stand  upon  the 
Earth,  but  in  the  sky,  a  vital  portion  not  only  of  our 
globe,  but  of  all  of  which  it,  too,  forms  part.  To  feel  it 
is  to  enter  upon  another  life ;  and  if  to  realization  of  its 
beauty,  its  grandeur,  and  its  sublimity  of  thought  these 
chapters  of  its  history  have  proved  in  any  wise  the 
portal,  they  have  not  been  penned  in  vain. 


NOTES 


NOTES 


METEOR  ORBITS 

If  the  space  of  the  solar  system  be  equally  filled  with 
meteors  throughout,  or  if  they  diminish  as  one  goes  out 
from  the  Sun  according  to  any  rational  law,  their  average 
speed  of  encounter  with  the  Earth  would  be  nearly  para- 
bolic. 

If  they  were  travelling  in  orbits  like  those  of  the  short- 
period  comets,  that  is  with  their  aphelia  at  Jupiter's  orbit 
and  their  perihelia  at  or  within  the  Earth's,  their  major 
axes  would  lie  between  6.2  and  5.2.  If  we  suppose  their 
perihelion  distances  to  be  equally  distributed  according  to 
distance,  we  have  for  the  mean  a  major  axis  of  5.7.  Their 
velocity,  then,  at  the  point  where  they  cross  the  Earth's 
track  would  be  given  by 


2.8S/ 

in  which  /i  =  i8-52  in  miles  per  second 

=  342.25, 
whence  v  =  23.76  in  miles  per  second. 

Suppose  them  to  be  approaching  the  Earth  indifferently 
from  all  directions. 

At  sunset  the  zenith  faces  the  Earth's  quit ;  at  sunrise 
the  Earth's  goal.  Let  0  be  the  real  angle  of  the  meteor's 
approach  reckoned  from  the  Earth's  quit ;  6l  the  apparent 
angle  due  to  compounding  the  meteor's  velocity-direction 

R  241 


242        THE   EVOLUTION   OF   WORLDS 

with  that  of  the  Earth.  Then  those  approaching  it  at  any 
angle  0  less  than  that  which  makes  01  =  90°  will  be  visible 
at  sunset ;  those  at  a  greater  angle,  at  sunrise.  The  angle 
#!  is  given  by  the  relation, 

/i  a 

cos  0,  ==  +-, 
x 

in  which  a  is  the  Earth's  velocity,  x  the  meteor's,  and  01 
is  reckoned  from  the  Earth's  quit. 

The  portion  of  the  celestial  dome  covered  at  sunset  is, 
therefore, 

/»0!    /»360° 

»/o    »/o 
where  <£  is  the  azimuth, 

/»180°    /*360° 

that  at  sunrise,  I       I   sin  0  -  dd  •  d$. 

If  the  meteors  have  direct  motion  only,  0  can  never 
exceed  90°,  and  the  limits  become, 

/»0X    /»360° 

for  sunset,  I      I   sin  6  -  dO  -  dd). 

i/O     %/§ 
/*90°    /»360° 

and  for  sunrise,  I   sin  0  -  dO  •  dd>. 

c/0      */0 

The  mean  inclination  at  sunset  is 

f*  jTt -sin  0-40 -dp 

«/a    */o 

«/o    Jo 

in  which  ^  must  be  expressed  in  terms  of  0,  etc. 

From  this  it  appears  that  the  relative  number  of  bodies, 
travelling  in  all  directions  and  at  parabolic  speed,  which 
the  Earth  would  encounter  at  sunrise  and  sunset  respect- 
ively would  be :  — 

sunrise 5-8 

sunset i.o 

and  with  the  speed  of  the  short-period  comets, 


NOTES 


243 


sunrise 8.0 

sunset i.o 

If,  however,  the  bodies  were  all  moving  in  the  same  sense 
as  the  Earth,  i.e.  direct,  the  ratios  would  be :  — 


PARABOLIC 
SPEED 

SPEED  OF  SHORT 
PERIOD  COMETS 

SPEED  OF  ACTUAL  SHORT- 
PERIOD  COMETS  ABOUT 
JUPITER 

2.4 

7.C 

7.7 

Sunset  

I.O 

I.O 

I.O 

As  the  actual  number  encountered  is  between  2  and  3  to  i, 
we  see  that  the  greater  part  must  be  travelling  in  the  same 
sense  as  the  Earth,  since  they  come  indifferently  at  all 
altitudes  from  the  plane  of  her  orbit. 


DENSITIES  OF  THE  PLANETS 

The  densities  of  the  principal  planets,  so  far  as  we  can 
determine  them  at  present,  the  density  of  water  being 
unity,  are :  — 

Mercury     .     .     .     3.65  Jupiter  ....  1.33 

Venus    ....     5.36  Saturn  ....  072 

Earth    ....     5.53  Uranus.     .     .     .  1.22 

Moon    ....    3.32  Neptune    .     .     .  i.n 

Mars     ....     3.93  mean  1.09 

mean  4.36      Sun 1.38 

The  second  decimal  place  is  not  to  be  considered  as 
anything  but  an  indication. 


VARIATION  IN  SPECTROSCOPIC  SHIFT 

In  the  case  of  a  body  reflecting  light,  the  shift  differs 
from  that  for  a  body  emitting  it.     If  the  planet  be  on  the 


244        THE   EVOLUTION  OF  WORLDS 

further  side  of  the  Sun,  the  approaching  rim  advances  both 
toward  the  Sun  and  toward  the  Earth,  thus  doubling  the 
shift.  The  receding  rim  recedes  in  like  manner.  At 
elongation  the  rims  approach  or  recede  with  regard  to  the 
Earth,  but  not  the  Sun,  and  the  shift  is  single  as  for 
emission.  At  inferior  conjunction  rotational  approach  to 
the  Earth  implies  rotational  recession  from  the  Sun,  and  the 
two  effects  cancel. 

4 

ON  THE  PLANETS'  ORBITAL  TILTS 

The  tilts  of  the  plane  of  rotation  of  the  Sun  and  of  the 
orbits  of  the  several  planets  to  the  dynamical  plane  of  the 
system  tabulated  are :  — 

Sun 7°  Asteroids various 

Mercury 6°  14'  Jupiter 20' 

Venus 2°  4'  Saturn 56' 

Earth i°  41'  Uranus i°  2' 

Mars 1°  38'  Neptune 43' 

where,  in  the  determination  of  that  plane,  the  latest  values 
of  the  masses  of  the  planets  and  the  rotations  of  the  Sun, 
Jupiter,  and  Saturn  have  been  taken  into  account. 

These  tilts  suggest  something,  doubtless,  but  it  is  by  no 
means  clear  what  it  is  they  suggest.  They  are  just  as  com- 
patible with  a  giving  off  from  a  slowly  condensing  nebula 
as  with  an  origin  by  shock.  The  greater  inclinations  of 
Mercury  and  Venus  may  be  due  to  their  late  birth  from 
the  central  mass  without  the  necessity  of  a  cataclysm,  the 
rotation  of  that  central  mass  out  of  the  general  plane  being 
caused  by  the  consensus  of  the  motions  of  the  particles  from 
which  it  was  formed.  The  accordance  of  the  larger  planet- 
ary masses  with  the  dynamical  plane  of  the  system  would 
necessarily  result  from  their  great  aggregations.  So  that 
this,  too,  is  quite  possible  without  shock. 


NOTES 


245 


PLANETS  AND  THEIR  SATELLITE  SYSTEMS 

If  we  compute  the  speeds  of  satellites  about  their  prima- 
ries in  the  solar  system  and  compare  them  with  the  veloci- 
ties in  their  orbits  of  the  planets  themselves,  a  striking 
parallelism  stands  displayed  between  the  several  systems. 
This  is  shown  in  the  following  table  of  them : 


MEAN  SPEED,  MILES 

PARABOLIC  SPEED 

A  SECOND 

AT  ORBIT 

RATIO  SPEED 

SAT.  ABOUT 

PRIMARY  TO 

of  Primary  in 

of  Satellite 

PLANET'S  SPEED 

Orbit 

about  Primary 

Miles  a  second 

IN  ORBIT 

V 

i' 

Jupiter 

. 

8.1 

II.5 

Sat. 

I 

10.7 

1.32 

2 

8.5 

1.05 

3 

6.7 

0.83 

4 

5-1 

0.63 

Saturn 

. 

6.0 

8.5 

I 

9.0 

1.50 

2 

7-9 

1-31 

3 

8.2 

1.36 

4 

6-3 

1.05 

5 

5-3 

0.89 

6 

3-5 

0.59 

8 

2.0 

0-34 

Uranus 

. 

4.2 

5-9 

i 

3-5 

0.82 

2 

2.9 

0.70 

3 

2-3 

0.54 

4 

2.0 

0.47 

Neptune   . 

3-4 

4.8 

I 

2.7 

0.81 

The  relations  here  disclosed  are  too  systematic  to  be  the 
result  of  chance. 


246        THE   EVOLUTION   OF  WORLDS 

The  orbits  of  all  these  satellites  have  no  perceptible 
eccentricity  independent  of  perturbation  except  lapetus,  of 
which  the  eccentricity  is  about  .03. 

In  view  of  the  various  cosmogonies  which  have  been 
advanced  for  the  genesis  of  the  solar  system  it  is  interest- 
ing to  note  what  these  speeds  imply  as  to  the  effect  upon 
the  satellites  of  the  impact  of  particles  circulating  in  the 
interplanetary  spaces  at  the  time  the  system  evolved.  To 
simplify  the  question  we  shall  suppose  —  which  is  suffi- 
ciently near  the  truth  —  that  the  planets  move  in  circles, 
the  interplanetary  particles  in  orbits  of  any  eccentricity. 

Taking  the  Sun's  mass  as  unity,  the  distance  R  of  any 
given  planet  from  the  Sun  also  as  unity,  let  the  planet's 
mass  be  represented  by  M  and  the  radius  of  its  satellite's 
orbit,  supposed  circular,  as  r.  We  have  for  the  space 
velocity  of  the  satellite  on  the  sunward  side  of  the  planet, 
calling  that  of  the  planet  in  its  orbit  V  and  that  of  the 
satellite  in  its  orbit  round  the  planet  vt 


For  a  particle,  the  semi-major  axis  of  whose  orbit  is  a^ 
and  which  shall  encounter  the  satellite, 

/    2  i\^ 

the  velocity  is       z/t  =  (  ---  1  • 
\R  -  r     aj 

That  no  effect  shall  be  produced  by  the  impact  of  these 
two  bodies,  their  velocities  must  be  equal,  or 


As   R  —  r  =  a^(  i  +  e)   for   the  point  of    impact   if   the 
particle  be  wholly  within  the  orbit  of  the  planet  and  e  the 


^          RM 

eccentricity  of  its  orbit,  we  find  e  =  2*^—  ---    -  approx. 


NOTES  247 

for  the  case  of  no  action,  the  other  terms  being  insensible 

r> 

for  the  satellites  in  the  table,  since  in  all  r  < 

400 

Supposing,  now,  the  particles  within  the  orbit  of  the 
planet  to  be  equally  distributed  according  to  their  major 
axes,  then  as  the  velocity  of  any  one  of  them,  taking 
R  —  r  =  R  approx.  as  unity,  is 


the  mean  velocity  of  all  of  those  which  may  encounter  the 
satellite  is,  at  the  point  of  collision, 


=  0.754; 

that  is,  just  over  three-quarters  of  the  planet's  speed  in  its 
orbit. 

If  we  suppose  the  particles  to  be  equally  distributed  in 
space,  we  shall  have  more  with  a  given  major  axis  in  pro- 
portion to  that  axis,  and  our  integral  will  become 


=  0.792  of  the  planet's  orbital  speed. 


248        THE   EVOLUTION  OF  WORLDS 


The  speed  v,  then,  at  which  a  satellite  must  be  moving 
round  the  planet  to  have  the  same  velocity  as  the  average 
particle  within  the  planet's  orbit,  is 

V  —  v±  =  v. 
This  velocity  is,  for  the  several  planets :  — 


DISTRIBUTION  OF 
PARTICLES  AS  THEIR 
MAJOR  AXES 

DISTRIBUTION  OF 
PARTICLES  EQUAL 
IN  SPACE 

Miles  a  second 

Miles  a  second 

Jupiter                                         • 

2  O 

i  6 

Sciturn 

T   c 

I  2 

Uranus 

1  •> 
I  O 

O  O 

Neptune  

0.8 

w.y 
O  7 

If  the  satellite  be  moving  in  its  orbit  less  fast  than  this, 
its  space-speed  will  exceed  that  of  the  average  particle ;  it 
will  strike  the  particle  at  its  own  rear  and  be  accelerated 
by  the  collision.  If  faster,  the  particle  will  strike  it  in 
front  and  retard  it  in  its  motion  round  its  primary. 

From  the  table  it  appears  that  all  the  large  satellites  of 
all  the  planets  have  an  orbital  speed  round  their  primaries 
exceeding  those  in  either  column.  In  consequence,  all  of 
them  must  have  been  retarded  during  their  formation  by 
the  impact  of  interplanetary  particles  and  forced  nearer 
their  primaries  than  would  otherwise  have  been  the  case ; 
and  this  whether  the  particles  were  distributed  more  densely 

toward  the  Sun,  as  — ,  or  were  equally  strewn  throughout. 

a\ 
For  interplanetary  particles  whose  orbits  lie  without  the 

particular  planet's  path  the  mean  speed  is  the  parabolic  at 
the  planet's  distance,  given  in  the  third  column  of  the  table. 
This  is  the  case  on  either  supposition  of  distribution.  The 


NOTES 


249 


orbital  speed  of  the  satellite  which  shall  not  be  affected  by 
collisions  with  them  is,  for  the  several  planets  :  — 


MILES  A  SECOND 

Jupiter    .     .     •     

•3   A 

Saturn                     •     •           •     .     .     . 

2  C 

**y 
1.7 

1  .4. 

All  the  satellites  but  lapetus  have  orbital  speeds  exceed- 
ing this,  and  consequently  are  retarded  also  by  these 
particles. 

For  particles  crossing  the  orbit  (2)  the  mean  velocity  would 
be  practically  parabolic,  1.4,  even  if  the  distribution  were  as 

^,  r1  being  the  distance  from  the  Sun.  The  effect  would  de- 
pend upon  the  angle  of  approach  and  in  the  mean  give  a 
greater  velocity  for  the  particle  than  for  the  satellite  within 
the  orbit,  a  less  one  without;  retarding  the  satellite  in 
both  cases.  Thus  the  total  effect  of  all  the  particles 
encountering  the  large  satellites  is  to  retard  them  and  to 
tend  to  make  them  hug  their  primary. 

For  retrograde  satellites  the  velocities  of  impact  with 
inside  and  outside  particles  moving  direct  are  respectively: 


INSIDE 

OUTSIDE 

Jupiter 

2  o  +  1} 

7/4-7/1 

Saturn    

I.I  4-  "V 

v  T  J'4 

V  4-  2  C 

Uranus  

I  .0  -|-  V 

V    T    *•} 

V  +  I  7 

Neptune     

08  +  7? 

"v  -f-  I  4 

In  both  cases  the  impact  tends  to  check  the  satellite. 
Comparing  with  these  the  velocities  of  impact  for  direct 
satellites  in  a  direct  plenum:  — 


250        THE   EVOLUTION   OF   WORLDS 


INSIDE 

OUTSIDE 

Jupiter   

2.O  —  V 

M  —  i) 

Saturn    

l.C    —  7/ 

2.c  —  i) 

Uranus  

I  O  —  If 

**3 

1.7  —  V 

Neptune     .     .          .          • 

0.8  —  v 

14  —  v 

the  signs  being  taken  positive  when  the  motion  is  direct, 
we  see  that  retrograde  satellites  would  be  more  arrested 
than  direct  ones  with  the  same  orbital  speed  round  the 
primary. 

In  a  plenum  of  direct  moving  particles,  then,  the  force 
tending  to  stop  the  satellite  and  bring  it  down  upon  the 
planet  is  greater  for  retrograde  satellites  than  for  direct 
ones. 

If,  therefore,  the  positions  of  the  satellites  have  been 
controlled  by  the  impact  of  interplanetary  particles,  the 
retrograde  satellites  should  be  found  nearer  their  planets 
than  the  direct  ones. 


ON  THE  INDUCED  CIRCULARITY  OF  ORBITS  THROUGH 
COLLISION 

Since  the  moment  of  momentum  is  the  velocity  into  the 
perpendicular  upon  its  direction,  in  the  time  dt  it  is  :  — 

vpdt  =  hdt  =  r*dO. 

The  whole  moment  of   momentum  from  perihelion  to 
perihelion  is  therefore  :  — 


tan 


< 

tan- 
2 


NOTES  251 


which  is  twice  the  area  of  the  ellipse. 

The  energy  in  the  ellipse  during  an  interval  dt  is 


-  mv*dt  =  -  mjJL  I \dt, 

2  2      ^\r     a) 


from  the  well-known  equation  for  the  velocity  in  a  focal 
conic.     The  integral  of  this  for  the  whole  ellipse  is 


2   h 

=  mfjfiirO*. 

Since 

C  C  a'1  -  <?  ja     2a-  i-e2          -  /   /I  -  e  B 

\rj0=\-  —xdQ=*  --  r-tarr1  "V—          tan.- 

•/  Ji+ecos0  (!_^2\i  V^i  +  ^  2 

and   j  f^dO  is  given  above. 

By  collision  a  part  of  this  energy  is  lost,  being  converted 
into  heat.  The  major  axis,  a,  is,  therefore,  shortened. 
But  from  the  expression  2  Tra2  •  (i  —  e2)^  for  the  moment  of 
momentum  we  see  that  this  is  greatest  when  e  is  least. 
If,  therefore,  a  is  diminished,  e  must  also  be  diminished,  or 
the  moment  of  momentum  would  be  lessened,  which  is 
impossible. 

7 

CAPTURE  OF  SATELLITES 

See  has  recently  shown  (Astr.  Nach.  No.  4341-42)  that 
a  particle  moving  through  a  resisting  medium  under  the 
attraction  of  two  bodies  revolving  round  one  another  in 
circles  may  eventually  be  captured  by  one  of  them  though 
originally  under  the  domination  of  both.  The  argument 
consists  in  introducing  the  effect  of  a  resisting  medium 


252        THE   EVOLUTION   OF  WORLDS 

upon  the  motion  in  the  space  permitted  by  Jacobi's  inte- 
gral, following  Darwin's  examination  of  this  space.  In 
the  actual  case  of  nature  the  effect  is  much  more  compli- 
cated, and  at  present  is  not  capable  of  exact  solution  for 
masses  other  than  indefinitely  small,  even  supposing 
circular  orbits  for  the  chief  bodies.  It  may,  however, 
explain  the  curious  relation  shown  in  the  arrangement  of 
the  direct  and  retrograde  movement  of  satellites. 


INDEX 


INDEX 


Abnormality,   the  survival  of  original 

state,  144,  146. 
Absorption  in  spectrum, 

planetary,  52,  161. 

of  Uranus,  118. 

of  Jupiter,  152. 

of  Saturn,  152. 
Achilles,  94. 
Adams,  119,  121. 
Adams,  Mr.  J.  C.,  123-126. 
Agassiz,  41. 
Airy,  121,  123. 
Albedo, 

of  dark  star,  27. 

of  Mercury,  62,  73-75. 

of  Venus,  73-75. 

of  Moon,  75. 

of  Jupiter,  104,  105. 

of  Saturn,  109. 

of  Uranus,  116. 

of  Neptune,  168. 

of  clouds,  195. 
Algol,  3. 

American  Academy,  125. 
Amphibians,  first  record  of,  188. 
Anderson,  Dr.  Thomas  D.,  8,  12. 
Andre,  265. 
Andromeda,  great  nebula  in,  10,  20,  21. 

constitution    disclosed     by    spectro- 
scope, 45,  48. 
Apex  of  Sun's  way,  26. 
Arago,  121. 
Asteroids,  39,  60,  61,  94-102. 

domain  of,  94. 

diminutive  size,  94,  101. 

number,  94,  101. 

peculiar  discovery  of,  95-98. 

never  formed  part  of  a  pristine  whole, 
98. 

where  thickest,  98. 

formation  of  large  planet  from,  pre- 
vented, 98,  99. 

mid-course     between     planets     and 
comets,  100. 


Asteroids  —  cont. 

shape  of,  101,  102. 

mammoth  meteorites,  102. 

mark  transition  between  inner  and 

outer  planets,  102. 
Atmosphere, 

spectrographic  study  of,  53,  54,  161. 

Mercury  deprived  of,  71,  75,  232. 

reflecting  power,  75. 

of  Venus,  75. 

Moon  deprived  of,  75,  232. 

thin  on  Mars,  75,  91,  232. 

of  Uranus,  enormous,  117,  118,  232. 

of  Neptune,  vast,  118,  232. 

of  Jupiter,  166,  232. 

depletion  of,  231-233. 

none  on  Ganymede,  232,  233. 

of  Saturn,  232. 

lacking  in  Saturn's  rings,  232. 
Avogadro,  228. 
Axes  of  planets, 

systematic  righting  of,  132. 

tilts  accounted  for,  146. 

B 

Babinet,  147. 

Backland,  68. 

Ball,  Sir  Robert,  145. 

Barrande,  M.,  178. 

Belopolski,  87. 

Bessel,  120,  121. 

Blandet,  M.,  175,  176. 

Bode,  95,  119. 

Bode's  law,  96,  100,  119,  122,  126. 

Bolometer,  194. 

Bolton,  Mr.  Scriven,  103,  105,  106. 

Boltzmann,  228. 

Bose,  157. 

Bouvard,  Alexis,  120,  121. 

Boyle,  228. 

Bradley,  68. 

C 

Cambrian  era,  178. 
Cambridge  Observatory,  123. 
Campbell,  9. 


255 


256 


INDEX 


Carboniferous  period,  179. 

Cassini,  76,  162. 

Celestial  mechanics,  28,  94,  155. 

Ceres,  101. 

Challis,  123. 

Chemistry,  indebted  to  the  stars,  160. 

Clausius,  228. 

Clerke,  Miss,  9,  164. 

Climate,  advent  of,  185. 

Clouds, 

none  on  Venus,  75. 

of  Jupiter  not  ordered  as  ours,  107, 
163,  167. 

Uranus  wrapped  in,  168. 

Neptune  wrapped  in,  168. 

Earth  once  wrapped  in,  170,  171,  178. 
Collision  of  dark  star  with  Sun,  25,  215. 

warning  of,  26-29. 

disturbances  previous  to,  29,  30. 

rarity  of  event,  30. 

Collisions    between    meteorites    of     a 
flock,  n,  49. 

causing  light,  49,  50. 
Columbus,  1 88. 
Comets,  33,  61. 

members  of  solar  system,  34,  35. 

orbits  of,  61,  100. 
Commensurability  of  orbital  period,  99, 

in. 
Congruities  of  solar  system,  128-137. 

deviations  from,  62,   100,   101,   130, 
131,  141. 

specify  mode  of  evolution,  137. 
Convection  currents,  219. 

in  atmosphere  of  Venus,  80. 
Copeland,  Dr.  7. 
Copernican  system,  58. 
Copernicus,  62. 
Cosmic  action,  i,  22,  184. 
Croll,  196. 
Cuticle  of  star,  effect  of  impact  on,  n. 


Dana,  177,  186,  189. 
Dark  stars, 

origin,  2. 

number,  2,  25. 

evidence  of,  3-5. 

collision  of,  10,  n. 

rendered  visible,  26. 
Darwin,  62,  138,  Notes  252. 
Day, 

lengthened  to  infinity,  70,  219. 


Day  —  cont. 

none  on  Venus,  83. 

Jovian,  163. 

first  appreciation  of,  186. 

coincides  with  month,  on  satellites,  225. 
Death  of  a  planet, 

defined,  214. 

catastrophic  cause,  215,  216. 

due  to  tidal  retardation  of  rotation, 
216-219. 

outcome  of  loss  of  oceans  and  air, 
226,  233. 

caused  by  extinction  of  Sun  itself,  234. 
Density, 

of  dark  star,  27. 

of  planets,  51,  Notes  243. 

of  Mercury,  63,  64. 

of  Venus,  90. 

of  Jupiter,  103,  117. 

of  Uranus,  115. 

Deserts,  increase  of,  on  Earth,  208-211. 
Devonian  era,  187. 
Dhurmsala  meteorite,  41. 
Diameter, 

of  Mercury,  63,  64,  66,  67. 

of  Venus,  90. 

of  Earth,  90. 

of  Mars,  91. 

of  satellites  of  Mars,  92. 

of  Jupiter,  103. 

of  Uranus,  115-117. 
Dust,  in  atmosphere  of  Venus,  75. 


Earth, 

characteristics,  not  universal,  90,  91, 

155- 

evolved  from  a  nebula,  149. 
internal  heat,  150. 
early  surface  temperature,  160,  169, 

170. 

once  cloud-wrapped,  170,  171,  178. 
solid  surface  formed,  171. 
hot  seas  of,  171,  172. 
self -sustained,  182. 
study  of,  within  province  of  astron- 
omy, 184. 

ceased  to  be  self-centred,  187. 
Sun    becomes    dominant    factor    in 

organic  life  of,  190. 
Earth  shine,  82. 
Eccentricity,  orbital, 

of  Mercury,  63,  65,  69,  222. 
of  asteroids,  erratic,  100,  101. 


INDEX 


257 


Eccentricity,  orbital  —  cont. 

of  satellites,  increases  with  distance 

from  primary,  134. 
Eclipsing  binaries,  3,  4. 
Ejectum  from  nova,  5,  16. 

rate  of  regression,  16. 
Elemental  substances,  159. 

in  Sun,  159. 

once  in  Earth,  160. 

discovery  of,  in  stars,  161,  162. 
Ellipticity, 

of  Jupiter,  103. 

of  Saturn,  109. 

of  Uranus,  115. 
Encke,  68. 
Energy, 

conservation  of,  140,  150,  151. 

dissipation,  140-142. 

conditions  for  a  minimum,  142. 
Eros,  fluctuation  of  light  of,  gives  evi- 
dence of  form,  101,  102. 
Evolution,  153. 

white  nebulae  in  process  of,  49. 

rounded  out,  56. 

of  solar  family,  100. 

evidence  of,  in  solar  system,  117. 

manner  of,  lessens  energy,  141. 
Evolution,  chemical,  155,  173. 

universal,  156. 

temperature  conducive  to,  157,  158. 

attendant  upon  cooling,  158,  162. 

steps  in,  shown  by  spectroscope,  161. 
Evolution,  physical,  155,  162. 

induced  by  cooling,  162. 


Fabry,  34. 

Fauna,  178,  179,  187. 

Faye,  175,  176. 

Flagstaff,  Arizona,  52,  66,  68,  79,  83, 

89,  92,  106,  no,  221,  232. 
clear  and  steady  air  of,  66,  86. 
Flamstead,  119. 
Fleming,  Mrs.,  7. 
Flemming,  120,  121. 
Flora,  of  paleologic  times,  177. 
French  Academy,  122. 


Galle,  Dr.,  122,  123,  125. 
Gases, 

peculiar  to  nebulae,  n,  16. 

occluded  in  meteorites,  42,  43. 

in  atmospheres  of  planets,  53-55. 


Gauss,  34,  96,  97. 
Geikie,  160,  177,  189. 
Geology, 

relation  to  astronomy,  173,  174,  183, 
184. 

scope  of,  174,  203. 

Geysers,  avenues  to  earlier  state,  160. 
Goodricke,  3. 

H 

Hakluyt,  188. 

Harvard  College  Observatory,  8,  12. 

Heat, 

molecular  motion,  150,  157,  230. 

the  result  of  evolving,  153. 

the  preface  to  higher  evolution,  153, 
156. 

laws  governing  amount  of,  190. 

atmosphere    keeps   out,    as   well    as 
stores,  191. 

effective,  received  from  Sun,  192-194. 

invisible  rays,  194. 

retained,  194—196. 

radiated,  194-196. 
Heat  of  condensation  of  Earth, 

accuses  concourse  of  particles,  151. 

evaluated,  151,  152. 

sufficient  for  geologic  phenomena,  152. 
Hector,  94. 
Helmholtz,  151. 
Hencke,  98. 

Herschel,  Sir  John,  122. 
Herschel,  Sir  William,  96,  114,  162. 
Hertha,  periodic  variability,  102. 
Hipparchus,  5. 
Holden,  9. 

Hubbard,  Professor,  124. 
Huggins,  52. 
Humphreys,  10. 
Huntington,  209. 


Ice  Age,  196. 

not  of  orbital  occasioning,  197-199. 
increased    precipitation,   the    cause, 

199,  200. 

a  local  affair,  200-202. 
Irradiation,  affecting  diameter  of  Mer- 
cury, 66,  68. 


Jacobi,  Notes  252. 
Julius,  Professor,  10. 
Juno,  101. 


258 


INDEX 


Jupiter,  103-108. 

not  solid,  104,  107. 

a    semi-sun,     105,    108,     152,     166, 
167. 

white  spots  of,  106. 
Jupiter,  "great  red  spot"  of,  164. 

time  of  rotation,  164. 

a  vast  uprush  of  heated  vapor,  165, 

1 66. 
Jupiter's  belts, 

secular  progression,  104. 

rotate  at  different  speeds,  104,  162, 
163. 

color,  104. 

wisps  across,  105,  106. 

bright  ones,  cloud,  163,  167. 

spectrographic  study  of,  166. 


Kapteyn,  14. 

Keeler,  19,  52,  no. 

Kepler,  6. 

Kinetic  theory  of  gases,  226,  228. 

corollary  of,  54. 

extension  of,  230,  231. 
Kirkwood,  Professor,  35. 


Lagrange,  94,  97- 

Lalande,  123,  124. 

Lane,  Homer,  234. 

Langley,  191,  194. 

Laplace,  34,  no,  127,  129,  131,    132, 

138,  139,  147,  152,  175- 
Laplacian  cosmos,  129,  130. 

false  congruities  of,  131-133. 

annular     genesis,     disproved,      138, 

!39- 

original    "fire  mist"  of,  impossible, 

138. 

Lapparent,  de,  173-176,  183,  189. 
Lemonnier,  115,  119. 
Leonard,  Miss,  79. 
Leverrier,  119,  121-126. 
Lexell,  115. 
Libration  in  longitude, 

of  Mercury,  65,  69,  70,  222,  223. 

causes  true  day,  70,  71. 

of  Venus,  inappreciable,  83,  223. 

of  Moon,  224. 
Lick  Observatory,  13,  14. 
Lockyer,  48. 
Lowell  Observatory,  65,  74. 


M 

Major  planets, 

gaseous,  117. 

constitution  of,  differs  from  Sun    or 
Earth,  161. 

types  of  early  planetary  stages,  162. 

self-centred  and  self-sustained,  168. 
Man,  immanent,  159. 
Mars, 

polar  caps,  198. 

canals  in  dark  regions,  206,  207. 

dying  of  exhaustion,  234. 
Mass, 

of  Mercury,  63,  64,  68. 

of  Mars,  91. 

of  Jupiter,  103. 

arrangement  of,  in  solar  system,  135- 

137,  J48. 
Massachusetts  Institute  of  Technology, 

134,  184. 
Mauvais,  125. 

Maxwell,  Clerk,  no,  113,  228. 
Mayer,  119,  151. 
Mendeleeff,  161. 
Mercury,  62-73. 

time  of  rotation  and  revolution  the 
same,  65,  69. 

axis  stands  plumb  to  orbit,  70. 

turns  same  face  to  the  Sun,  70,  72, 
134,  221. 

surface  markings,  72,  221. 

color,  72. 
Meteorites,  31,  35,  36. 

cosmic  bodies,  32,  33. 

relation  to  shooting  stars,  36. 

members  of  solar  system,  36. 

composition,  40-44,  55. 

fused   by   friction  with  atmosphere, 
40. 

temperature,  41,  55. 

fragments  of  a  dark  body,  44. 

link  past  to  present,  44,  56,  57,  130. 
Meteors, 

orbits  of,  36,  39,  Notes  241-243. 

visibility  of,  38. 
Meteor-streams,  33,  61. 

first  recognition  of,  34. 

disintegrated  comets,  34. 
Michelson,  10. 
Milham,  Professor,  99. 
Mira  Ceti,  235. 
Mohler,  10. 
Molecular  speeds,  gaseous,  228-231. 

critical  velocity,  230,  231. 


INDEX 


259 


Molecule,  organic,  power  in    its  insta- 
bility, 1 60. 
Moment  of  momentum,  140,  Notes  250. 

cause  of  original,  130. 
Moment    of    momentum,   conservation 
of,  140. 

applied  to  solar  system,  141-143. 
Momentum,  140. 
Monch,  Mr.,  10. 
Moon, 

turns  same  face  to  Earth,  134,  208, 
224,  225. 

once  fiery,  now  dead,  233,  234. 
Mountains,  none  on  Mars,  91. 
Muller,  73,  74,  104,  105,  116. 

N 

Naval  Observatory  at  Washington,  122. 
Nebulae, 

origin  of,  10,  n. 

amorphous,  18,  44. 

planetary,  18. 

spectrum  of  amorphous,  45. 
Nebulae,  spiral,  17-25,  44. 

evolved  from  disrupted  stars,  10-15. 

relation  to  novae,  14-16. 

corpuscular  character  of,  15,  16. 

knots  and  patches  of,  15. 

most  common,  19,  20. 

two- armed,  20,  25. 

central  neucleus,  globular,  21. 

not  due  to  explosive  action,  22,  23,  25. 

not  caused  by  disintegration,  24,  25. 

cause  of  development,  24,  25. 

spectrum  of,  45-48. 

composed  of  flocks  of  meteorites,  48, 
49. 

constitution    established    by  spectro- 
scope, 49,  50. 
Nebular  hypotheses,  173. 
Neologic  times,  clearing  of  sky  in,  185. 
Neptune,  118. 

rotates  backward,  118. 

owes  discovery  to  mathematical  tri- 
umph, 119-126. 

faint  belts  on,  168. 

further  advanced  than  giant  planets, 

168. 

Newcomb,  67. 
Newton,  Professor,  36,  42. 
Newton,  Sir  Isaac,  34. 
Nova  Aurigae,  7,  8,  12. 

history  chronicled   by   its  spectrum, 
8,9. 


Nova  Cygni,  7. 
Novae,  6,  7. 

origin  5,  10. 

first  chronicled,  5. 

spectroscopic  study  of,  7. 
Nova  Persei,  7. 

history  of,  12-15. 


Oceans. 

none  on  Mars,  91. 

evaporation  of,  204. 

basins  of,  on  Moon,  204-208. 

basins  of,  on  Mars,  206,  207. 
Olbers,  97. 

Olmstead,  Professor,  33. 
Orbital  distance, 

of  Mercury,  62. 

of  Venus,  73. 

of  Mars,  91. 

of  Eros,  94. 

of  Saturn,  108. 
Orbital  tilts, 

of  asteroids,  erratic,  100,  101. 

of  satellites  of  Uranus,  1 16. 

of    planets,   substantially  the  same, 
129-131,  Notes  244. 

deviation  from  rule,  by  Mercury,  131. 

of '  satellites,   increase  with    distance 

from  primary,  133,  134. 
Orbits, 

determining  factors,  35. 

rendered  more  circular  by  collisions, 
141-143,  Notes  250,  251. 

made  more   conformant  to    general 

plane  by  collisions,  141-143. 
Orion,  great  nebula  in,  18. 


Paleologic  times, 

much  warmth  and  little  light  in,  172. 
fallacies  in  geologists'    expositions  of, 

174-176. 

climate  continuous,  177,  186. 
seas  warm,  177,  178. 
explained  by  cloud  envelope,  178. 
corroboration    of    explanation,    187, 

179. 

excessive  rain  in,  185,  186. 
passage    into    Neologic,    essentially 

astronomic,  185. 
Pallas,  101. 
Parabolic  speed  at  orbit,  Notes  245. 


26o 


INDEX 


Patroclus,  94. 
Peirce,  no,  125,  126. 
Perrine,  15. 
Perrotin,  116. 
Perturbations, 

in   motion   of   planets,    heralding    a 

catastrophe,  28,  30. 
reflected,  63. 

mass  of  planet  determined  by,  68. 
of  asteroids  by  Jupiter,  98,  99. 
restrictive  action  of,  99. 
the  fashioning  force  of  planetary  or- 
bits, 99,  100. 
of  rings  of  Saturn  by  satellites,  in, 

112. 

of  Uranus  lead  to  discovery  of  Nep- 
tune, 121-126. 
Petersen,  Dr.,  123. 
Photometric  determinations,  92,  93. 
background,  the  fundamental  factor 

in,  92,  93. 
Piazzi,  96.  ' 
Pilgrim  Star,  5,  6. 
Planetary  astronomy,   advance  in,   59, 

60. 
Planetology,  203. 

denned,  173,  174. 
Planets,  61. 

knots  in  spiral  nebulae,  25,  139. 
developed    by    agglomeration,     143, 

149,  151,  152. 
Pliny,  5- 

Plutonic  rocks,  160. 
Pluvial    eras,    contemporaneous    with 

glacial,  200. 
Polyp     corals,    in     paleologic     times, 

186. 

Pristine  motion  of  planetary  particles, 
retrograde,  144. 
superfluous  energy  in,  145. 
unstable,  145. 
Ptolemaic  system,  58. 


R 


Refrigeration,    tempered    by    loss    of 

cloud,  196. 
Revolutions, 

of  shooting  stars,  39. 
of  asteroids,  direct  like  planets,  100. 
planetary,  in  same  sense,  129,  130. 
outermost  satellites,  retrograde,  132. 
of    satellites    explained      146,     147, 
Notes  252. 


Ritchey,  14. 

Roberts,  Dr.,  20. 

Roche,  Edouard,  no. 

Rosse,  Lord,  17. 

Rotation  of  planets,  131,  132. 

systematic  righting  of  axes,  132. 

initially,  retrograde,  146. 
Rotation  period, 

of  Venus,   spectrographically  deter- 
mined, 83,  85-90. 

of  Mars,     spectrographically     deter- 
mined, 88,  89. 

of    Jupiter,    spectrographically    de- 
termined, 89. 

of  Uranus,  116. 
Royal  Observatory,  Edinburgh,  7. 


Satellites,  61. 

of  Mars,  92. 

of  Saturn,  108,  112. 

of  Uranus,  116. 

solid,  117. 

of    Neptune,  118 

turn  same  face  to  primaries,  134,  147, 
148,  225. 

latest  discoveries  in  regard  to  mo- 
tions of,  146. 

origin  of,  147. 

death  of,  before  planet,  233. 

impact  of  interplanetary  particles  on, 
Notes  246-250. 

capture  of,  Notes  251,  252. 
Saturn,  108-114. 

belts  of,  109,  1 68. 

inherent  light,  109,  152. 
Saturn's  rings,  109-114. 

mechanical  marvel  of,  not  early  ap- 
preciated, no. 

discrete  particles,  no,  135. 

knots  upon,  110-113. 

not  flat,  but  tores,  111-114. 

show  devolution  —  not  pristine  state 
of  solar  system,  138,  139. 

once  a  congeries,  139. 
Schaeberle,  9. 
Schiaparelli,  34,  36,  64-66,  69,  76,  77, 

221. 

Schroeter,  65,  77. 
Seasons, 

loss  of,  71,  83,  217,  218. 

begin  with  clearing  of  sky,  185. 

fully  developed,  189. 
See,  Notes  251. 


INDEX 


261 


Seeliger,  10. 
Shooting-stars,  33,  35. 

radiant  of,  33,  36. 

members  of  solar  system,  36-40. 

tiny  planets,  39. 
Siderite,  36. 
Silurian  era,  178. 

Sirona,  periodic  variability  of,  102. 
Sky,  cause  of  clearing,  187. 
Slipher,  Dr.  V.  M.,  52,  79,  83,  86,  88, 

89,  117,  161,  166. 
Slipher,  Mr.    E.  C.,  79,  233. 
Solar  constant,  191. 
Solar  system, 

evolved  from  a  dark  star,  44. 

evidence  of  origin,  51,  130. 

characteristics  of,  60-62. 

evolutionarily  one,  62. 

gap   in    progression   of   orbital    dis- 
tances, 95—100. 

bodies  of,  egg-shaped,  217. 
Specific  gravity,  of  stone  and  iron,  44. 
Spectroscope,  7,  84. 
Spectroscopic  shift,  84. 

determining  velocity,  3. 

in  Nova  Aurigae,  9. 

produced  by  great  pressure,  10,  13. 

produced    by  anomalous   refraction, 
10. 

produced  by  change  of  density,  10, 

I3\ 
explained,  85. 

variation  in,  Notes  243,  244. 
Spectrum, 

of  Nova  Persei,  12,  13. 
nebular,  13,  16,  45-48. 
peculiarities  of  nebular,  explained, 

5°- 
photographic  extension  of,   52,   117, 

161. 

of  major  planets,  52,  53,  161. 
of  belts  of  Jupiter,  166. 
Spiral  structure,  implies  rotation  com- 
bined with  motion  out  or  in,  22. 
Stability  of  a  system,  condition  for,  140, 

141. 

Stoney,  Dr.  Johnstone,  231. 
Struve,  109. 
Suess,  179. 
Sun, 

original  slow  rotation  of  the,  130. 
heat  of,  234,  235. 
reversion  to  a  dark  star,  235,  236. 
Sun  spots,  104,  1 66. 


Temperature, 

of  Moon,  191,  192. 
of  Mars,  192,  194,  196. 
defined,  230. 

no  such  thing  as,  in  space,  230. 
Tercidina,      periodic      variability     of, 

1 02. 
Tertiary  times,  entrance  of  color  with, 

189,  190. 

Tidal  action,  143-147,  216-218. 
causes  loss  of  energy,  144. 
inoperative,  144,  145,  147. 
changes  retrograde  rotation  of  planet 

to  direct,  145-147,  217. 
on  satellites,  147. 
slows  down  spin,  148,  217. 
brings    plane    of    rotation    down    to 

orbital  plane,  217. 
lengthens  day  to  infinity,  219. 
analytically  expressed,  224. 
greatest  on  planets  near  Sun,    135, 

224. 
Tidal  action,  disruptive,  130. 

exemplified    by   spiral    nebulae,    24, 

25- 

hinted  at,  by  meteorites,  55. 
theory  corroborated   by  densities  of 

planets,  51. 
theory  corroborated  by  atmospheres 

of  planets,  52-55. 
on  comets,  139. 
cause  of  Saturn's  rings,  139. 
Tisserand,  68. 
Titius,  95. 
Todd,  68. 
Trees,  deciduous,  first  appearance  of, 

189. 

Trilobites,  blindness  of,  178,  179. 
Twining,  33. 
Tycho  Brahe,  5. 


U 


Uranus,  114-118. 

history  of  discovery,  114,  115,  119. 

a  ball  of  vapor,  115,117. 

belts  of,  115,  116,  168. 

tilt  of  axis  to  ecliptic,  great,  115. 

spectroscopic    revelations    of,     117, 

118. 

in  an  early  amorphous  state,  118. 
further    advanced    than    the    giant 
planets,  168. 


262 


INDEX 


Velocity, 

of  Mercury  in  orbit,  63. 

of    satellites   about   primary,   Notes 

245- 

of  major  planets,  in  orbit,  Notes  245. 
Venus,  73-90. 

surface    markings,    74,    77,    79,    80, 

83,  220,  221. 

brilliancy    due    to    cloudless    atmos- 
phere, 75. 
importance  of   rotation   period,    75, 

76. 
turns  same  face  to  the  Sun,  77-80, 

134,  220,  221. 
ice  on  the  night  side,  causes  ashen 

light,  82. 

Very,  Professor,  16,  191,  192,  194. 
Vesta,  10 1. 
Vogel,  52. 

Volcanoes,  avenues  to  earlier  state,  160. 
Von  Zach,  96. 


W 


Walker,  Mr.,  123,  124. 
Water, 

becoming  more  scarce,  203,  204,  211. 

lacking  on  Moon,  204. 
Water-vapor, 

in  atmosphere  of  Jupiter,  53. 

in  atmosphere  of  Mars,  91,  161. 

smaller  planet  has  less  hold  on,  207. 
Williams,  Mr.  Stanley,  103. 
Witt,  de,  94. 
Wolf,  Dr.,  13. 
Wolf,  Max,  94. 
Wolf-Rayet  stars,  13,  48. 
Wright,  13,  43. 


Year,  of  Uranus,  116. 
Yerkes  Observatory,  232. 
Young,  46. 


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Transcript. 

"  Whether  or  not  we  choose  to  follow  the  author  of  this  book  to  his 
ultimate  inferences,  he  at  least  opens  up  a  field  of  fascinating  con- 
jecture. The  work  is  written  in  a  style  as  popular  as  the  precise 
enumeration  of  the  ascertained  facts  permits,  and  if  the  narrative  is  not 
in  all  its  details  as  entrancing  as  a  novel,  it  nevertheless  transports  us 
into  a  region  of  superlatively  romantic  interest."  —  New  York  Tribune. 

"  No  doubt  the  highest  living  authority  on  Mars  and  things  Martian  is 
Prof.  Percival  Lowell,  director  of  the  observatory  at  Flagstaff,  Arizona, 
an  astronomical  investigator  and  writer  known  over  the  entire  world. 
Professor  Lowell's  book,  '  Mars  and  Its  Canals,'  is  the  final  word,  up  to 
the  present,  on  the  planet  and  what  we  know  of  it."  —  Review  of 
Reviews. 


PUBLISHED    BY 

THE   MACMILLAN   COMPANY 

64-66  Fifth  Avenue,  New  York 


PERCIVAL  LOWELL'S 

Mars  as  the  Abode  of  Life 

Illustrated,  8vo,  $2.50  net 

The  book  is  based  on  a  course  of  lectures  delivered  at  the  Lowell 
Institute  in  1906,  supplemented  by  the  results  of  later  observations. 
It  is,  in  the  large,  the  presentation  of  the  results  of  the  author's  re- 
search into  the  genesis  and  development  of  what  we  call  a  world  ;  not 
the  mere  aggregating  of  matter,  but  the  process  by  which  that  matter 
comes  to  be  individual  as  we  find  it.  He  bridges  with  the  new  science 
of  planetology  the  evolutionary  gap  between  the  nebular  hypothesis 
and  the  Darwinian  theory. 

"  It  is  not  only  as  an  astronomer  but  as  a  writer  that  Professor  Lowell 
charms  the  reader  in  this  work.  The  beguilement  of  the  theme  is 
well  matched  by  the  grace  and  literary  finish  of  the  style  in  which  it  is 
presented.  The  subject  is  one  to  beget  enthusiasm  in  its  advocates, 
and  the  author  certainly  is  not  devoid  of  it.  The  warmth  and  earnest- 
ness of  the  true  lover  of  his  theme  shine  through  the  entire  work  so 
that  in  its  whole  style  and  illustrations  it  is  a  charming  production." 

—  St.  Louis  Globe  Democrat. 

"  Mr.  Lowell  approaches  the  subject  by  outlining  the  now  generally 
accepted  theory  of  the  formation  of  planets  and  the  solar  system.  He 
describes  the  stages  in  the  life  history  of  a  planet  three  of  which  are 
illustrated  in  the  present  state  of  the  earth,  Mars,  and  the  moon.  He 
tells  what  conditions  we  would  expect  to  find  on  a  planet  in  what  we 
may  call  the  Martian  age,  and  proceeds  to  show  how  the  facts  revealed 
by  observation  square  with  the  theories.  The  book  is  fascinatingly 
readable."  —  The  Outlook. 

"  So  attractive  are  the  style  and  the  illustrations  that  the  work  will 
doubtless  draw  the  attention  of  many  new  readers  to  its  fascinating 
subject.  Professor  Lowell  has  fairly  preempted  that  portion  of  the 
field  of  astronomy  which  interests  the  widest  readers,  for  there  is  no 
doubt  that  speculation  regarding  the  possibility  of  life  on  other  planets 
than  our  own  has  a  peculiar  attraction  for  the  average  human  mind. 
.  .  .  For  the  convenience  of  the  non-technical  reader,  the  body  of  the 
book  has  been  made  as  simple  and  understandable  as  possible." 

—  Philadelphia  Press. 


PUBLISHED    BY 

THE    MACMILLAN    COM  PANT 

64-66  Fifth  Avenue,  New  York 


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